refactor: compact pill-based filter toolbar

Merge the toggleable multi-line FilterBar and the separate ActiveFilterChips
strip into a single always-visible row of pills. Each filter category is a
pill that opens a small popover with its underlying control; when active, the
pill shows its current value inline (so the chips strip is redundant).

- New FilterPill primitive: outside-click + Escape to close, optional inline
  X to clear without opening the popover.
- FilterBar rebuilt out of pills for Date/Type/Rating/Color/Flag/Tags/Sort,
  with a Clear-all pill on the right when any filter is active.
- Drop filterBarOpen from filterStore, the SlidersHorizontal toggle from
  TopBar, the \\ shortcut from useKeyboardShortcuts, and the matching hint
  from KeyboardHints — the bar is always visible now.
- Delete ActiveFilterChips; its information lives inside the pills.

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

frontend/src/components/KeyboardHints.tsx

@@ -21,7 +21,6 @@ export function KeyboardHints() {
         { key: 'Click', action: 'Select' },
         { key: 'Shift+Click', action: 'Range' },
         { key: 'Space', action: 'Preview' },
-        { key: '\\', action: 'Filters' },
         { key: '/', action: 'Search' },
       ]