Add RetroArch save sync with the RomM server

[gantoine]

Summary

Implements save synchronization between local games and the RomM server, per the save sync RFC, the server-side #3137, and the behaviour change in #3479.

Uses the server's API sync mode:

1. Register this machine as a RomM device once (POST /api/devices, client playnite, sync_mode: "api"); the server-assigned device_id is persisted in settings.
2. POST /api/sync/negotiate — report the local save (rom_id, file name, MD5 content_hash, updated_at, size) and let the server decide per save: upload / download / conflict / no_op.
3. Execute the returned operations — POST/PUT /api/saves (multipart saveFile), GET /api/saves/{id}/content then POST /api/saves/{id}/downloaded.
4. POST /api/sync/sessions/{id}/complete.

Conflicts are resolved most-recent-wins (the RFC default), with the decision logged.

Scope & behaviour

• RetroArch battery saves (.srm): the local save path is derived from retroarch.cfg (savefile_directory plus the sort_savefiles_enable / sort_savefiles_by_content_enable options and the : base-dir token), with a recursive-search fallback for sorted layouts.
• Saves only — save states aren't part of the server's device-sync flow.
• When: pull the newest save before launch (OnGameStarting), push after play (OnGameStopped), plus a "Sync saves with RomM" game-menu action.
• Gated behind a new "Enable save sync" setting (off by default).
• Save content is hashed with MD5 to match the server's comparison exactly.
• Failures are logged and surfaced via notifications; they never block a game from launching.

Files

• New: Saves/SaveSyncService.cs (orchestration), Saves/RetroArchConfig.cs + Saves/SaveFileHash.cs (pure, unit-tested), Models/RomM/Save/* (wire models).
• Changed: RomM.cs (play hooks + menu item), Settings/Settings.cs + SettingsView.xaml (new toggle, persisted device id).
• Tests: RetroArchConfigTests, SaveFileHashTests.

Testing

• Added unit tests for the pure logic (retroarch.cfg parsing, save-path resolution, MD5 hashing).
• Not built/run locally — the plugin is WPF / .NET Framework 4.6.2 (Windows-only) and this environment has no Windows SDK; relying on CI for the build + test run.

Open questions

• Device registration needs the client token to carry device/asset scopes — worth documenting.
• extension.yaml version bump and a README section for the new setting can follow if wanted.

🤖 Draft opened for review.

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