Sync: surface partial failures via syncError instead of silent SYNC_BOARDS_SUCCESS

[RodriSanchez1]

Part of #2195.

Problem

syncBoards() (src/components/Board/Board.actions.js:790) always dispatches SYNC_BOARDS_SUCCESS when its outer try block completes, but every per-board operation in pushLocalChangesToApi (:759, :780) and every deletion verification in applyRemoteChangesToState (:603-613) swallows its own errors with console.error. As a result, the top-level syncError state is null even when half the batch failed, and the UI has no way to know that some boards are still out of sync.

Impact

• Users get no signal that a board failed to upload.
• Other gaps in this epic (communicator desync, markToUpdate orphans) become invisible because the global status reads "success".
• Debugging in production relies entirely on console.error lines.

Proposed direction

• Collect per-board failures inside pushLocalChangesToApi and applyRemoteChangesToState into a structured result (e.g. { failed: [{ boardId, phase, error }] }).
• Dispatch SYNC_BOARDS_SUCCESS with that payload, or a new SYNC_BOARDS_PARTIAL_SUCCESS action, so the reducer can populate syncError / a new syncWarnings field.
• Update the UI mapping table in docs/sync-engine.md §12 to include the partial-failure state.

Acceptance criteria

• Failing one board push leaves state.board.syncError (or equivalent) populated with which boards failed.
• All other boards still complete normally.
• No regression: a fully successful sync still produces a clean state.