feat: By Date sidebar — year/month drilldown navigation

The "By Date" library node was decorative. Now it's a real
hierarchical navigator: expand to see year buckets (with photo
counts), expand a year to see its months, click any year or
month to filter the timeline to that date range.

Backend
- New GET /library/date_buckets aggregates non-discarded photos
  by year+month from Photo.taken_at via SQLite strftime, returning
  [{ year, count, months: [{ month, count }] }] sorted newest-
  first. NULL taken_at rows are excluded.

Frontend
- New library.dateBuckets() helper + DateBucketYear / Month types.
- New hooks/useDateBucketsQuery.ts with a 60s staleTime.
- LeftSidebar builds the By Date subtree dynamically from the
  query: each year is a tree node with month children. Year nodes
  use the Calendar icon, months render as their full English name.
- applyLibraryNode handles two new id prefixes:
  'date-year-{year}'  → setDateFrom YYYY-01-01, setDateTo YYYY-12-31
  'date-month-{Y}-{M}'→ setDateFrom YYYY-MM-01, setDateTo YYYY-MM-LL
  where LL is the last day of the month (computed via Date trick
  new Date(year, month, 0).getDate() — uses month-day=0 to roll
  back into the previous month's last day).
- isItemActive recognises when the current dateFrom/dateTo matches
  a year or month node so the sidebar selection highlight stays
  in sync with the filter store (also when filters are set
  externally via the filter bar or URL hydration).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/hooks/useDateBucketsQuery.ts

@@ -0,0 +1,12 @@
+import { useQuery } from '@tanstack/react-query'
+import { library, type DateBucketYear } from '../services/api'
+
+export const DATE_BUCKETS_QUERY_KEY = ['library', 'date_buckets'] as const
+
+export function useDateBucketsQuery() {
+  return useQuery<DateBucketYear[]>({
+    queryKey: DATE_BUCKETS_QUERY_KEY,
+    queryFn: library.dateBuckets,
+    staleTime: 60_000,
+  })
+}