fix(sync): dashboard sync covers current year plus Nov-Dec of previous

/sync-bank now uses one shared default window in both implementations:
1 November of the previous year to 31 December of the current year.
Payments dated around New Year are picked up in January; duplicates
are still filtered by Sync ID.

Python used a hardcoded 2026 window and would have stopped syncing on
2027-01-01; Go used the current calendar year only.

Adds make sync-year [YEAR=N] with the same window. The CLI default
without flags (last 30 days) is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/internal/services/banksync/sync.go

@@ -36,6 +36,18 @@ type SyncOpts struct {
 	PrintFioTable bool      // with DryRun: print every fetched Fio txn with NEW/DUP status
 }
 
+// DefaultWindow is the window used by the dashboard sync: the whole current
+// calendar year plus the last two months of the previous one, so payments
+// dated around New Year are still picked up in January. Re-fetching is safe
+// because rows are deduplicated by Sync ID.
+// Mirrors default_sync_window in legacy/scripts/sync_fio_to_sheets.py.
+func DefaultWindow(now time.Time) (from, to time.Time) {
+	yr := now.Year()
+	from = time.Date(yr-1, time.November, 1, 0, 0, 0, 0, time.UTC)
+	to = time.Date(yr, time.December, 31, 23, 59, 59, 0, time.UTC)
+	return from, to
+}
+
 // SyncToSheets fetches Fio transactions from all provided clients and appends
 // new ones to the payments sheet. Returns the number of rows appended.
 // Ports legacy/scripts/sync_fio_to_sheets.py sync_to_sheets.