fix: assorted UI polish from review pass

- FilterPill: drop the inline value text from the active state. Pills
  now stay the same width whether or not a filter is set; the popover
  is the canonical place to read the value, and the title attribute
  surfaces it on hover.
- TopBar: remove the search input — search lives in the filter bar now.
- FilterBar: add a search input on the left, with the pill cluster
  centered between it and a flex-shrink-0 Clear-all on the right.
- LeftSidebar / HeapsPanel: count badges use a fixed-width slot
  (h-5 min-w-[24px], tabular-nums) so counts line up in the same
  visual column across rows. Empty rows reserve the slot.
- LeftSidebar: pull section counts (All Photos, Rated, Duplicates,
  Discarded) from a new useLibraryStatsQuery hook backed by the
  expanded /library/stats endpoint. Tags count was already wired.
- backend/library: stats endpoint returns per-section counts that
  match the filter the sidebar applies on click.
- Stats invalidation hooked into the standard photo-mutation paths.
- RightSidebar header: h-12 to match TopBar height.
- Timeline sticky date overlay: only show once the natural in-grid
  header has scrolled OUT of the viewport. Avoids the duplicate-label
  flash when both labels would be visible.

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

frontend/src/components/timeline/Timeline.tsx

@@ -257,13 +257,17 @@ export function Timeline() {
     return () => el.removeEventListener('scroll', onScroll)
   }, [])
 
-  // Find the latest header whose start <= scrollTop. That's the label of
-  // the group containing whatever is currently at the top of the viewport.
+  // Find the latest header whose BOTTOM is above the viewport top. That's
+  // the group whose natural in-grid header has scrolled out of view —
+  // exactly the case where we want to pin the label as a sticky overlay.
+  // If the natural header is still visible (scrolled but not yet past),
+  // we return null and let the in-grid label do the work, avoiding the
+  // duplicate-label flash.
   const stickyLabel = useMemo(() => {
     if (headerOffsets.length === 0) return null
     let current: string | null = null
     for (const h of headerOffsets) {
-      if (h.offset <= scrollTop) current = h.label
+      if (h.offset + HEADER_HEIGHT <= scrollTop) current = h.label
       else break
     }
     return current