fix(sync-report): the last background sync reports how it ended

[juanky201271]

Problem

The last-background-sync card on the sync report took its state from the error field alone. Anything without one showed a green check and Finished OK, including:

• a task the system killed before it wrote its end time (dateEnd = 0, message "Starting OK.")
• a task cancelled because the app was in the foreground, or with no wallet (message ends in "KO.", no error)
• a sync skipped in offline mode

Tapping View/Report Error also cleared the error and rewrote background.json, so the card flipped to green after one report.

Change

• backgroundOutcome in syncModel.ts derives one of ok | skipped | failed | interrupted from the file the native task writes: failed on an error or a message ending in "KO.", interrupted when there is no end time, skipped on the offline message, ok otherwise.
• The card shows icon, colour and label per outcome. Red with View/Report Error and a tappable row only when there is error text; amber Did not finish; grey Skipped, offline; green Finished OK.
• Reporting an error no longer clears it. setBackgroundSyncErrorInfo and BackgroundFileImpl.writeBackground had no other caller and are removed; only the native task writes the file now.
• Three new sync.* keys in the five languages.

Tests

Five unit cases on backgroundOutcome, one per outcome plus the KO-without-error one. The screen snapshot is unchanged.

🤖 Generated with Claude Code

[juanky201271]

Moved into #1643.