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/notes/facets.ts

@@ -0,0 +1,56 @@
+// Convert between the board's URL query and a NoteFacets object. The URL query IS the
+// filter state (so a lens is shareable/bookmarkable and a saved view is just a link).
+import type { LocationQuery, LocationQueryRaw } from "vue-router";
+import type { NoteFacets } from "../stores/notes";
+
+function one(v: LocationQuery[string]): string | undefined {
+  return (Array.isArray(v) ? v[0] : v) ?? undefined;
+}
+
+export function facetsFromQuery(q: LocationQuery): NoteFacets {
+  const rawLabel = q.label;
+  const labels = (Array.isArray(rawLabel) ? rawLabel : [rawLabel]).filter(
+    (x): x is string => typeof x === "string" && x.length > 0,
+  );
+  const f: NoteFacets = {};
+  const text = one(q.q);
+  if (text) f.q = text;
+  const color = one(q.color);
+  if (color) f.color = color;
+  const kind = one(q.kind);
+  if (kind === "text" || kind === "list") f.kind = kind;
+  if (labels.length) f.label = labels;
+  if (one(q.has_reminder) === "true") f.has_reminder = true;
+  if (one(q.has_attachment) === "true") f.has_attachment = true;
+  const after = one(q.created_after);
+  if (after) f.created_after = after;
+  const before = one(q.created_before);
+  if (before) f.created_before = before;
+  return f;
+}
+
+export function facetsToQuery(f: NoteFacets): LocationQueryRaw {
+  const q: LocationQueryRaw = {};
+  if (f.q) q.q = f.q;
+  if (f.color) q.color = f.color;
+  if (f.kind) q.kind = f.kind;
+  if (f.label?.length) q.label = f.label;
+  if (f.has_reminder) q.has_reminder = "true";
+  if (f.has_attachment) q.has_attachment = "true";
+  if (f.created_after) q.created_after = f.created_after;
+  if (f.created_before) q.created_before = f.created_before;
+  return q;
+}
+
+// How many facets are active (labels counted individually) — drives the "Filters (N)" badge.
+export function facetCount(f: NoteFacets): number {
+  let n = 0;
+  if (f.q) n++;
+  if (f.color) n++;
+  if (f.kind) n++;
+  n += f.label?.length ?? 0;
+  if (f.has_reminder) n++;
+  if (f.has_attachment) n++;
+  if (f.created_after || f.created_before) n++;
+  return n;
+}