revert: drop By Date sidebar node and supporting code

Tried it, didn't add value beyond what the date-grouped timeline
already gives. The grouped, sticky-headered timeline (which kicks
in by default whenever sortBy is taken_at) is the better
affordance for date navigation — duplicating that as a sidebar
drilldown was just clutter.

Removes the full stack:
- LeftSidebar: by-date tree node, byDateChildren computation,
  date-year-/date-month- handlers in applyLibraryNode, isItemActive
  branches that matched a date-range filter, the now-unused
  setDateFrom/setDateTo/filterDateFrom/filterDateTo selectors, and
  the Calendar icon import.
- frontend/src/hooks/useDateBucketsQuery.ts deleted entirely.
- api.ts: library.dateBuckets helper and DateBucketYear/Month types.
- backend/app/routers/library.py: GET /library/date_buckets endpoint
  and its strftime aggregation query.

The dateFrom/dateTo filter state stays in filterStore — the
FilterBar still uses it for the "Date" range inputs.

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

frontend/src/services/api.ts

@@ -113,16 +113,6 @@ export const photos = {
 }
 
 // Library API
-export interface DateBucketMonth {
-  month: number
-  count: number
-}
-export interface DateBucketYear {
-  year: number
-  count: number
-  months: DateBucketMonth[]
-}
-
 export const library = {
   scan: async () => {
     const response = await api.post('/library/scan')
@@ -138,12 +128,6 @@ export const library = {
     const response = await api.get('/library/stats')
     return response.data
   },
-
-  /** Year/month aggregation for the sidebar By Date drilldown. */
-  dateBuckets: async (): Promise<DateBucketYear[]> => {
-    const response = await api.get('/library/date_buckets')
-    return response.data
-  },
 }
 
 // Heaps API