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>

backend/app/routers/library.py

@@ -47,50 +47,6 @@ 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"""