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

src/thoughtsync/saved_filters.py

@@ -20,7 +20,7 @@ _ALLOWED_PARAM_KEYS = {
     "q",
     "color",
     "kind",
-    "labels",
+    "label",  # matches the repeatable ?label= query param (stored as an array)
     "has_reminder",
     "has_attachment",
     "created_after",