M6 1902b: facet bar + saved views UI (frontend)

The dead-simple facet bar over the board + saved views in the sidebar,
completing task 1902. Filter state lives in the URL query, so a filtered
board is a shareable lens and a saved view is just a link ("one space,
many lenses").

- notes/facets.ts: facetsFromQuery / facetsToQuery / facetCount helpers.
- FilterBar.vue (board only): a "Filters (N)" toggle expanding to text
  search + color swatches + label chips + has-reminder / has-attachment /
  Lists / Notes toggles + a created-date range; Clear + "Save view".
  Each control writes the URL query (router.replace).
- notes store: load(view, label, facets) builds the query; NoteFacets type
  + activeFacets; import reload preserves active facets.
- savedFilters store + sidebar "Views" section (each a query-link, delete
  on hover); loaded on mount.
- BoardView derives facets from the query, reloads on facet change (ignores
  ?open=), and shows a "no notes match these filters" empty state.
- Backend: saved-filter param whitelist keys on `label` (matches the
  repeatable ?label= query) so saved views keep their labels. New filter
  icon.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/Icon.vue

@@ -28,6 +28,7 @@ const paths: Record<string, string> = {
   device: '<rect width="14" height="20" x="5" y="2" rx="2" ry="2"/><path d="M12 18h.01"/>',
   paperclip: '<path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l8.57-8.57A4 4 0 1 1 18 8.84l-8.59 8.57a2 2 0 0 1-2.83-2.83l8.49-8.48"/>',
   link: '<path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"/><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"/>',
+  filter: '<polygon points="22 3 2 3 10 12.46 10 19 14 21 14 12.46 22 3"/>',
   copy: '<rect width="14" height="14" x="8" y="8" rx="2" ry="2"/><path d="M4 16c-1.1 0-2-.9-2-2V4c0-1.1.9-2 2-2h10c1.1 0 2 .9 2 2"/>',
 };
 </script>