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/layout/RightSidebar.tsx

@@ -150,7 +150,7 @@ export function RightSidebar() {
     const id = activePhotoId ?? selectedPhotos[0]
     return (
       <div className="flex h-full flex-col bg-surface">
-        <div className="flex items-center justify-between border-b border-border px-4 py-3">
+        <div className="flex h-12 flex-shrink-0 items-center justify-between border-b border-border px-4">
           <h2 className="text-sm font-semibold text-text">Photo Details</h2>
           <button
             onClick={clearSelection}