perf+style: grid subscription hygiene, a11y, shadcn-style consistency

Perf / a11y (high-impact review items)
- Timeline arrow-key handler binds once per (viewMode, currentSection)
  and reads fresh state via navStateRef instead of an 8-element dep
  array of new-each-render values.
- usePhotosQuery collapses 14 individual Zustand selectors into one
  useShallow selector returning the params object.
- PhotoThumbnail no longer subscribes to the search query directly;
  Timeline subscribes once and passes it down as a prop.
- PhotoThumbnail gains role="button", tabIndex, aria-label, aria-pressed,
  Enter/Space key handlers and a focus-visible ring. Timeline marked
  role="grid"; RightSidebar marked role="region".

Style consistency
- Swap clsx for cn (tailwind-merge aware) across 17 files so
  conflicting utility classes collapse correctly.
- New Badge primitive (ui/badge.tsx) with default/neutral/overlay/
  outline variants; adopted in ColorsView, RatedView, TagsView for
  the repeated count overlay pill.
- Fix palette drift: text-amber-400 -> text-star, text-green-*
  -> text-pick, text-red-* -> text-reject (5 files).
- Button gains an xs size (h-6 px-1.5 text-[11px]) for the repeated
  compact-button pattern.

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

frontend/src/components/admin/UserManagement.tsx

@@ -105,11 +105,7 @@ export function UserManagement() {
               </td>
               <td className="py-1.5 pr-4">
                 <span
-                  className={
-                    u.is_active
-                      ? 'text-green-400'
-                      : 'text-red-400'
-                  }
+                  className={u.is_active ? 'text-pick' : 'text-reject'}
                 >
                   {u.is_active ? 'Active' : 'Inactive'}
                 </span>
@@ -129,7 +125,7 @@ export function UserManagement() {
                     <Button
                       variant="ghost"
                       size="icon"
-                      className="h-6 w-6 hover:text-red-400"
+                      className="h-6 w-6 hover:text-reject"
                       onClick={() => setDeactivatingUser(u)}
                       title="Deactivate user"
                     >