build: migrate to ESLint 9 flat config

[chrisae9]

Summary

• migrate from unsupported ESLint 8 and eslint-config-airbnb-base to ESLint 9 flat configuration
• preserve Airbnb correctness behavior with supported core and Stylistic replacements
• lint all first-party JavaScript and Vue files with separate browser, test, and Node globals
• enforce zero lint warnings and clean up existing Vue/test lifecycle findings

Dependency status

• removes every deprecated direct dependency from the install graph
• remaining deprecation notices are upstream transitives from Babel CLI and Vue Test Utils
• major framework/runtime upgrades remain separate migration PRs

Verification

• clean install, production build, 153 Node tests, 235 Vitest tests, warning-free whole-repository lint, zero-vulnerability audit, and clean diff check
• Mac mini ARM64, exact commit de193d5345d2233a928321f25c965fd704155393: clean install, production build, 153 Node tests, 235 Vitest tests, warning-free whole-repository lint, zero-vulnerability audit, and clean diff check
• independent review found and verified bounded cleanup for signal-terminated child processes; CodeRabbit incremental review is currently rate-limited

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Review details

⚙️ Run configuration

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Run ID: 63f4f409-2178-4c19-850f-4f6e47db604f

📥 Commits

Reviewing files that changed from the base of the PR and between 5358b04 and de193d5.

⛔ Files ignored due to path filters (1)

• package-lock.json is excluded by !**/package-lock.json, !package-lock.json

📒 Files selected for processing (18)

• .eslintrc.js
• eslint.config.mjs
• package.json
• server.js
• src/__tests__/vuetify3-migration.test.js
• src/components/PlexItem.vue
• src/components/PlexMediaPlayDialog.vue
• src/components/PlexThumbnail.vue
• src/components/TheSidebarRight.vue
• src/views/PlexLibrary.vue
• src/views/RoomJoin.vue
• src/views/WebPlayer.vue
• test/cache.test.js
• test/combineurl.test.js
• test/kick.test.js
• test/server.test.js
• test/socket-validation.test.js
• vite.config.js

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