feat(i18n): add vue-i18n with en, uk, de, ru, zh-CN locales

[TimeToBuildBob]

Summary

Consolidates i18n work from PRs #855 (vue-i18n with uk/de/ru) and #865 (zh-CN translations) into a single clean PR on latest master, per Erik's request (see comments on #895).

Includes:

• vue-i18n@8 infrastructure with TypeScript locale files
• Language picker in Settings with localStorage persistence
• Browser locale detection on first visit
• 5 locale files: en, uk, de, ru, zh-CN
• Core views migrated to $t() calls (Header, Buckets, Home, Timeline, Activity, Settings, Footer, and more)
• Locale validation script and unit tests
• Author attribution preserved: NureRykushBohdan (uk/de/ru) and JhihJian (zh-CN)

Follow-up (not in scope):

• Additional view migrations (remaining components still use hardcoded strings)
• Category Builder, Alerts, Graph, and dev-only views (tracked as gaps in feat(i18n): add Ukrainian, German, and Russian UI translations #855)

Closes #855, #865
Supersedes #895

[greptile-apps]

Greptile Summary

This PR adds full vue-i18n v8 infrastructure to aw-webui, migrating core views (Header, Footer, Home, Buckets, Timeline, Activity, Settings, and more) from hard-coded English strings to $t() calls, with locale files for en, uk, de, ru, and zh-CN.

• Infrastructure: src/i18n/index.ts wires up VueI18n, browser-locale detection, localStorage/backend persistence via setAppLocale, and moment locale sync; LanguageSettings.vue exposes a working language picker in Settings \u2192 General.
• Locale files: Five complete locale files (412 keys each) with consistent key structure; validated by the new npm run check:locales script that checks key parity and placeholder consistency against the English baseline.
• Test coverage: Unit tests for isAppLocale, browser detection, localStorage restore, and settings-store locale loading round-trips are included and cover the main code paths.

Confidence Score: 5/5

The change is safe to merge; it adds an opt-in i18n layer without altering any existing data paths or store contracts.

All five locale files are fully wired up including zh-CN, the settings store locale lifecycle is correct, and the new functionality is well-covered by unit tests. The only gaps are cosmetic: document.documentElement.lang is not set when the locale comes from browser detection alone, and the installed vue-i18n v8 carries a deprecation notice — neither affects runtime correctness.

The browser-detection-only path in src/i18n/index.ts and src/stores/settings.ts is the one place where the document.documentElement.lang attribute does not get updated.

Important Files Changed

Filename	Overview
src/i18n/index.ts	New i18n module: registers VueI18n, exports isAppLocale, getInitialLocale, setAppLocale, and the i18n instance with all 5 locales wired up. Browser locale detection and localStorage persistence work correctly; moment locale is synced. Minor gap: document.documentElement.lang is not set during browser-detection-based init.
src/stores/settings.ts	Adds locale to the settings state and integrates it into the load/save lifecycle. Invalid locales from server and localStorage are validated and warned on. Server locale takes priority over localStorage, which takes priority over browser detection. The only subtle gap is that setAppLocale is not called in the browser-detection-only path.
src/views/settings/LanguageSettings.vue	New settings panel with a dropdown for all 5 supported locales. Reads from the store, validates via isAppLocale, and calls both update (persists to backend) and setAppLocale (updates i18n, moment, DOM, localStorage) on change. Loading guard prevents stale display.
scripts/check-locales.mjs	New validation script that checks key parity, placeholder consistency, and untranslated strings across all 5 locale files against the en baseline. Uses new Function to parse static TS object literals (intentional, with eslint-disable comment). Hooked up as npm run check:locales.
src/components/UncategorizedNotification.vue	Migrated to i18n: period labels now use translation keys, and the body string is built via $t('uncategorized.body', { duration, percent, period }). Replaced `
src/i18n/locales/zh-CN.ts	Simplified Chinese locale, now properly imported and registered in index.ts and selectable in the language picker. Key structure matches en.ts. Included in check-locales.mjs validation.
test/unit/i18n.test.js	Unit tests covering isAppLocale, localStorage-based restore, browser locale detection (including zh-CN and fallback to en), and setAppLocale side-effects. Tests are thorough and use module isolation correctly.
test/unit/store/settings.locale.test.js	Integration tests for locale loading in the settings store: valid/invalid server locale, valid/invalid localStorage locale, and the sync with the live i18n instance. Covers the main code paths correctly.
src/main.js	Imports the i18n instance from the new module and passes it to the root Vue constructor. Clean integration with no side-effects.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Browser
    participant i18n as i18n/index.ts
    participant Store as settings.ts
    participant LS as localStorage
    participant Server as aw-server

    Browser->>i18n: module init
    i18n->>LS: getItem('locale')
    alt stored locale found
        LS-->>i18n: 'de'
        i18n->>i18n: "initialLocale = 'de'"
    else no stored locale
        i18n->>Browser: navigator.language
        Browser-->>i18n: 'de-DE'
        i18n->>i18n: detectBrowserLocale() returns 'de'
    end
    i18n->>i18n: "new VueI18n({ locale })"
    i18n->>i18n: moment.locale('de')

    Browser->>Store: settings.load()
    Store->>Server: get_settings()
    Server-->>Store: "{ locale } or {}"
    alt locale in server or localStorage
        Store->>i18n: setAppLocale('de')
        i18n->>LS: setItem('locale', 'de')
        i18n->>Browser: "document.lang = 'de'"
    else browser-detected only
        Store->>Store: "patch({ locale: i18n.locale })"
        Note over Store,Browser: document.lang NOT updated here
    end

    Browser->>Store: user picks 'ru' in LanguageSettings
    Store->>i18n: setAppLocale('ru')
    i18n->>LS: setItem('locale', 'ru')
    i18n->>Browser: "document.lang = 'ru'"
    Store->>Server: POST /0/settings/locale 'ru'

Loading

%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant Browser
    participant i18n as i18n/index.ts
    participant Store as settings.ts
    participant LS as localStorage
    participant Server as aw-server

    Browser->>i18n: module init
    i18n->>LS: getItem('locale')
    alt stored locale found
        LS-->>i18n: 'de'
        i18n->>i18n: "initialLocale = 'de'"
    else no stored locale
        i18n->>Browser: navigator.language
        Browser-->>i18n: 'de-DE'
        i18n->>i18n: detectBrowserLocale() returns 'de'
    end
    i18n->>i18n: "new VueI18n({ locale })"
    i18n->>i18n: moment.locale('de')

    Browser->>Store: settings.load()
    Store->>Server: get_settings()
    Server-->>Store: "{ locale } or {}"
    alt locale in server or localStorage
        Store->>i18n: setAppLocale('de')
        i18n->>LS: setItem('locale', 'de')
        i18n->>Browser: "document.lang = 'de'"
    else browser-detected only
        Store->>Store: "patch({ locale: i18n.locale })"
        Note over Store,Browser: document.lang NOT updated here
    end

    Browser->>Store: user picks 'ru' in LanguageSettings
    Store->>i18n: setAppLocale('ru')
    i18n->>LS: setItem('locale', 'ru')
    i18n->>Browser: "document.lang = 'ru'"
    Store->>Server: POST /0/settings/locale 'ru'

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_{Reviews (3): Last reviewed commit: "fix(i18n): fix lint error - replace empt..." | Re-trigger Greptile}

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.42%. Comparing base (17d4a4b) to head (aea2642).

Files with missing lines	Patch %	Lines
src/stores/settings.ts	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #896      +/-   ##
==========================================
+ Coverage   35.58%   38.42%   +2.84%     
==========================================
  Files          36       42       +6     
  Lines        2161     2225      +64     
  Branches      401      436      +35     
==========================================
+ Hits          769      855      +86     
+ Misses       1371     1292      -79     
- Partials       21       78      +57     

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[TimeToBuildBob]

@greptileai review

[TimeToBuildBob]

✅ Ready to Merge

All checks passing:

• ✅ CI green (all 8 jobs pass)
• ✅ Greptile 5/5 confidence — "Safe to merge"
• ✅ Review threads resolved (zh-CN locale wired, TypeScript null guard, and duplicate entries all addressed in follow-up commits)
• ✅ Merge status: CLEAN

Summary: This PR consolidates i18n work (vue-i18n@8 with en, uk, de, ru, zh-CN) from PRs #855 and #865 into a single clean implementation. The changes are additive, fall back cleanly to English on any missing key, and all five locale files are fully wired and validated.

Ready for merge by maintainer.

[TimeToBuildBob]

@greptileai review