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/components/layout/LeftSidebar.tsx

@@ -16,10 +16,12 @@ import clsx from 'clsx'
 import { AddSourceFolderDialog } from '../dialogs/AddSourceFolderDialog'
 import { sourceFolders, library, photos as photosApi } from '../../services/api'
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
+import { format } from 'date-fns'
 import { toast } from '../ToastContainer'
 import { useFilterStore } from '../../store/filterStore'
 import { HeapsPanel } from '../heaps/HeapsPanel'
 import { PHOTO_DRAG_MIME } from '../timeline/PhotoThumbnail'
+import { useDateBucketsQuery } from '../../hooks/useDateBucketsQuery'
 
 interface TreeItem {
   id: string
@@ -41,7 +43,12 @@ export function LeftSidebar() {
   const setRatingMin = useFilterStore((s) => s.setRatingMin)
   const setFlag = useFilterStore((s) => s.setFlag)
   const setFolderId = useFilterStore((s) => s.setFolderId)
+  const setDateFrom = useFilterStore((s) => s.setDateFrom)
+  const setDateTo = useFilterStore((s) => s.setDateTo)
   const filterFolderId = useFilterStore((s) => s.folderId)
+  const filterDateFrom = useFilterStore((s) => s.dateFrom)
+  const filterDateTo = useFilterStore((s) => s.dateTo)
+  const { data: dateBuckets = [] } = useDateBucketsQuery()
   const [dropTargetId, setDropTargetId] = useState<string | null>(null)
 
   // Bulk discard mutation for the drag-onto-Discarded interaction.
@@ -115,6 +122,26 @@ export function LeftSidebar() {
           const folderId = id.slice('folder-'.length)
           clearAllFilters()
           setFolderId(folderId)
+        } else if (id.startsWith('date-year-')) {
+          const year = parseInt(id.slice('date-year-'.length), 10)
+          if (Number.isFinite(year)) {
+            clearAllFilters()
+            setDateFrom(`${year}-01-01`)
+            setDateTo(`${year}-12-31`)
+          }
+        } else if (id.startsWith('date-month-')) {
+          // Format: date-month-YYYY-MM
+          const parts = id.slice('date-month-'.length).split('-')
+          const year = parseInt(parts[0], 10)
+          const month = parseInt(parts[1], 10)
+          if (Number.isFinite(year) && Number.isFinite(month)) {
+            const lastDay = new Date(year, month, 0).getDate()
+            const mm = String(month).padStart(2, '0')
+            const dd = String(lastDay).padStart(2, '0')
+            clearAllFilters()
+            setDateFrom(`${year}-${mm}-01`)
+            setDateTo(`${year}-${mm}-${dd}`)
+          }
         }
     }
   }
@@ -184,6 +211,22 @@ export function LeftSidebar() {
     setExpandedItems(newExpanded)
   }
   
+  // Build the dynamic By Date subtree from the date_buckets aggregation.
+  // Year nodes contain month nodes; clicking either sets the date range
+  // filter (handled in applyLibraryNode). Sorted newest-first by the
+  // backend already.
+  const byDateChildren: TreeItem[] = dateBuckets.map((year) => ({
+    id: `date-year-${year.year}`,
+    label: String(year.year),
+    icon: <Calendar className="h-4 w-4" />,
+    count: year.count,
+    children: year.months.map((m) => ({
+      id: `date-month-${year.year}-${m.month}`,
+      label: format(new Date(year.year, m.month - 1, 1), 'MMMM'),
+      count: m.count,
+    })),
+  }))
+
   const libraryTree: TreeItem[] = [
     {
       id: 'library',
@@ -191,7 +234,12 @@ export function LeftSidebar() {
       icon: <HardDrive className="h-4 w-4" />,
       children: [
         { id: 'all-photos', label: 'All Photos', icon: <Image className="h-4 w-4" />, count: 0 },
-        { id: 'by-date', label: 'By Date', icon: <Calendar className="h-4 w-4" /> },
+        {
+          id: 'by-date',
+          label: 'By Date',
+          icon: <Calendar className="h-4 w-4" />,
+          children: byDateChildren,
+        },
         { id: 'rated', label: 'Rated', icon: <Star className="h-4 w-4" />, count: 0 },
         { id: 'discarded', label: 'Discarded', icon: <Trash2 className="h-4 w-4" />, count: 0 },
       ],
@@ -213,7 +261,7 @@ export function LeftSidebar() {
   // Derive whether a tree item is currently the "active" filter target.
   // Folder rows are selected when the filter store's folderId matches; the
   // library "All Photos" virtual node is selected when no folder/heap filter
-  // is set.
+  // is set; year/month nodes when their date range matches.
   const isItemActive = (id: string): boolean => {
     if (id.startsWith('folder-')) {
       return filterFolderId === id.slice('folder-'.length)
@@ -221,6 +269,18 @@ export function LeftSidebar() {
     if (id === 'all-photos') {
       return filterFolderId === null && selectedItem === 'all-photos'
     }
+    if (id.startsWith('date-year-')) {
+      const year = id.slice('date-year-'.length)
+      return filterDateFrom === `${year}-01-01` && filterDateTo === `${year}-12-31`
+    }
+    if (id.startsWith('date-month-')) {
+      const parts = id.slice('date-month-'.length).split('-')
+      const year = parseInt(parts[0], 10)
+      const month = parseInt(parts[1], 10)
+      if (!Number.isFinite(year) || !Number.isFinite(month)) return false
+      const mm = String(month).padStart(2, '0')
+      return filterDateFrom === `${year}-${mm}-01` && filterDateTo?.startsWith(`${year}-${mm}-`) === true
+    }
     return selectedItem === id
   }