Handle SyncState.CONFLICT — surface conflict to user and allow resolution

[jmuci]

Context

Currently, sync conflicts are resolved silently server-side (last-write-wins or server wins). This is acceptable for v1 but leaves users unaware that their local edits may have been overwritten.

Desired Behavior

When the server rejects a local change due to a conflict:

1. Mark the recipe as syncState = CONFLICT in Room
2. Show the user a non-intrusive indicator (e.g. a yellow badge on the recipe card)
3. Provide a conflict resolution UI: 'Your version' vs 'Server version' with Keep/Overwrite options

Notes

This is acknowledged as future work — implementing the SyncState.CONFLICT DB state is the key first step. The UI resolution flow can follow.

Acceptance Criteria

• SyncState.CONFLICT is a valid Room-persisted state
• Sync worker sets this state when server returns a conflict response
• Recipe list shows a visual indicator for conflicted recipes
• (Future) Resolution UI allows choosing local vs server version