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>

backend/app/routers/library.py

@@ -47,6 +47,50 @@ async def trigger_scan():
     
     return {"status": "success", "message": "Library scan started"}
 
+@router.get("/date_buckets")
+async def get_date_buckets(db: AsyncSession = Depends(get_db)):
+    """Aggregate non-discarded photos by year and month based on taken_at.
+    Used by the LeftSidebar 'By Date' subtree to render a year-then-month
+    drilldown without fetching every photo. Returns:
+        [{ year: 2026, count: 5, months: [{ month: 4, count: 5 }] }]
+    """
+    year_expr = func.strftime('%Y', Photo.taken_at)
+    month_expr = func.strftime('%m', Photo.taken_at)
+
+    result = await db.execute(
+        select(
+            year_expr.label('year'),
+            month_expr.label('month'),
+            func.count(Photo.id).label('count'),
+        )
+        .where(
+            Photo.is_discarded == False,  # noqa: E712
+            Photo.taken_at.is_not(None),
+        )
+        .group_by(year_expr, month_expr)
+        .order_by(year_expr.desc(), month_expr.desc())
+    )
+    rows = result.all()
+
+    years: dict[int, dict] = {}
+    for year_str, month_str, count in rows:
+        if not year_str or not month_str:
+            continue
+        try:
+            year = int(year_str)
+            month = int(month_str)
+        except ValueError:
+            continue
+        bucket = years.setdefault(
+            year, {'year': year, 'count': 0, 'months': []}
+        )
+        bucket['count'] += int(count)
+        bucket['months'].append({'month': month, 'count': int(count)})
+
+    # Already ordered by the SQL ORDER BY (newest first), preserve insertion.
+    return list(years.values())
+
+
 @router.get("/scan/status")
 async def get_scan_status(db: AsyncSession = Depends(get_db)):
     """Get current scan status"""