Migrate save sync to use server-side negotiate API

[gantoine]

Replaces client-side save sync resolution with the (under development) server-side negotiate API (POST /api/sync/negotiate) from rommapp/romm#3137.

What changed

Before: grout fetched remote saves per-ROM (N concurrent GET /api/saves calls), ran its own conflict detection by comparing local mtimes against DeviceSyncs.LastSyncedAt, and separately discovered remote-only saves.

After: grout scans local saves, computes MD5 content hashes, and sends everything to the server in a single negotiate call. The server determines upload/download/conflict/no_op actions and returns a sync session. Grout executes the plan and marks the session complete.

Changes

• romm/sync.go (new): API types and client methods for negotiate/complete endpoints
• romm/saves.go: Added session_id to upload/download queries. + moved MarkDeviceSynced/ConfirmSaveDownloaded
  • Server now handles device sync state automatically via device_id on uploads and optimistic=true on downloads
• internal/fileutil/fileutil.go: Added ComputeMD5 matching the server's content hash algorithm
• sync/flow.go (rewritten): ResolveSaveSync now calls negotiate instead of doing client-side resolution, and ExecuteSaveSync tracks session lifecycle
• sync/execute.go (extracted from flow.go): Upload/download execution with session ID pass-through
• sync/scan.go (extracted from flow.go): Local save scanning and slot selection

Removed

• Client-side conflict detection (determineAction, DetermineActions)
• Per-ROM remote save fetching (FetchRemoteSaves, DiscoverRemoteSaves`)
• Helper functions (LocalSavesWithoutRemote, NewSaveUploadActions)
• MarkDeviceSynced / ConfirmSaveDownloaded / endpointSaveDownloaded

Requires rommapp/romm#3137