feat: add login and sync commands

[grauschnabel]

Summary

• immich-go login: Interactive credential storage to ~/.config/immich-go/config.yaml with server validation and API key verification
• immich-go sync down: Download server assets to local directory with checksum-based state tracking, atomic writes, and YYYY/YYYY-MM/ directory structure
• immich-go sync up: Upload local assets to server with robust dedup (SHA1 → name+date±5s+size matching via shared assetmatch package)
• Config layering: Global config → local config → CLI flags, with flat-key fallback so server/api-key from global config work for all subcommands
• Delete safeguards: Only tracked assets deleted, confirmation for >10 deletions, --force to bypass
• Graceful Ctrl+C: Saves state and releases lock on interrupt, resumable on next run
• Dry-run: No state written, all actions logged to stdout
• Documentation: New docs/commands/login.md and docs/commands/sync.md, updated README, command reference, and configuration docs

Test plan

• go build ./... passes
• go test -race ./internal/syncstate/... — 10 tests pass (incl. CaptureDate migration + roundtrip)
• go test -race ./internal/assetmatch/... — 13 tests pass (dedup decision matrix)
• golangci-lint run — 0 new issues
• Manual: immich-go login → credentials saved and reused by subsequent commands
• Manual: immich-go sync down -d /tmp/test --dry-run → shows planned downloads on stdout
• Manual: immich-go sync down -d /path → downloads assets, creates state
• Manual: Ctrl+C during sync → state saved, lock released, resumes on next run
• Manual: immich-go sync up -d /path
• Manual: immich-go sync down --delete --dry-run

🤖 Generated with Claude Code

[simulot]

Thanks for the PR and for contributing to immich-go.

To help me better understand the context of these changes, could you please provide a brief overview of your motivation and the specific use case this addresses?

In the meantime, please consider the following technical requirements for this PR:

Branching: All PRs should be targeted against the develop branch rather than main.

End-to-End (e2e) Testing: While unit tests are necessary, e2e tests are welcomed to validate logic against a live test server. Please add new test cases in ./internal/e2e/client.

Edge Cases: Ensure you account for scenarios where Immich handles multiple assets with identical filenames and capture dates.

Cross-Platform Validation: These tests run on both Linux and Windows runners to catch OS-specific bugs, such as improper usage of path vs. filepath.

Once these items are addressed, I'll be happy to do a deeper dive into the code. Looking forward to your updates!

[grauschnabel]

Motivation & Use Cases

Why login + sync?

login solves a friction point: every immich-go invocation currently requires --server and --api-key flags. For users who run sync regularly (cron jobs, backup scripts), this is tedious. login stores credentials in ~/.config/immich-go/config.yaml with server validation and API key verification, so subsequent commands just work.

sync down enables offline backups: download your entire Immich library to a local directory with YYYY/YYYY-MM/filename organization. State tracking (.immich-sync/state.json) ensures idempotent operation — running it twice only downloads new assets.

sync up enables bidirectional workflows: edit photos locally (Lightroom, etc.), then push changes back to the server. Uses the same robust dedup logic as the upload command (SHA1 → filename+date±5s+size matching).

Changes in this update (addressing review feedback)

1. Shared dedup logic (internal/assetmatch/): Extracted the matching algorithm from app/upload/advice.go into a reusable package. Both upload and sync up now share the same AdviceCode types and CompareDate() function, ensuring consistent dedup behavior.

2. Richer sync up matching: Previously sync up only checked SHA1. Now it uses the full 2-phase algorithm (checksum → name+date+size), correctly handling cases like re-encoded photos with different checksums.

3. CaptureDate in sync state: AssetEntry now stores capture_date for smarter matching across sync sessions. Backward compatible with omitzero.

4. Dry-run + delete fix (previous commit): confirmDeletion() skipped in dry-run, summary messages show [dry-run] prefix.

5. E2E tests (internal/e2e/client/sync_test.go): 4 test cases covering sync up/down basics and idempotency.

6. PR base → develop per project branching conventions.