feat(gallery): faceted refine panel UI (Phase 2 frontend)

Add a 'Refine ▾' toggle to the gallery filter bar that expands a full-width
GalleryFacetPanel below it, inside the same sticky hazey chrome. The panel
offers platform chips (with live counts + a 'No platform' unsourced bucket),
two count-badged curation-flag toggles (Untagged / No artist), and a from/to
date range bounded by the facet min/max.

Store gains the platform/untagged/no_artist/date_from/date_to filter params
(URL-mirrored, AND-composed) and a panel-gated, single-flighted loadFacets()
that fetches /api/gallery/facets scoped to the active filter. Shared
cloneFilter/filterToQuery helpers keep the bar and panel writing one URL
format. The panel auto-opens on deep-link when refine filters are present and
refetches counts (debounced) on every filter change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/test/components/galleryFacetPanel.spec.js

@@ -0,0 +1,53 @@
+// @vitest-environment happy-dom
+import { describe, it, expect, vi, afterEach } from 'vitest'
+
+import GalleryFacetPanel from '../../src/components/gallery/GalleryFacetPanel.vue'
+import { useGalleryStore } from '../../src/stores/gallery.js'
+import { freshPinia, mountComponent } from '../support/mountComponent.js'
+
+const FACETS = {
+  total: 5,
+  platforms: [
+    { value: 'patreon', count: 3 },
+    { value: null, count: 2 },  // unsourced → "No platform" bucket
+  ],
+  untagged: 4,
+  no_artist: 1,
+  date_min: '2020-01-01T00:00:00+00:00',
+  date_max: '2026-06-01T00:00:00+00:00',
+}
+
+function stubFetchOk(body) {
+  globalThis.fetch = vi.fn(async () => ({
+    ok: true, status: 200, statusText: 'OK',
+    text: async () => JSON.stringify(body),
+  }))
+}
+
+describe('GalleryFacetPanel', () => {
+  afterEach(() => vi.restoreAllMocks())
+
+  it('renders platform options with counts and the curation flags', () => {
+    const pinia = freshPinia()
+    stubFetchOk(FACETS)              // covers the onMounted loadFacets() call
+    const s = useGalleryStore()
+    s.facets = FACETS               // seed so counts render synchronously
+    const w = mountComponent(GalleryFacetPanel, { pinia })
+    const t = w.text()
+    expect(t).toContain('patreon')
+    expect(t).toContain('3')
+    expect(t).toContain('No platform')
+    expect(t).toContain('Untagged')
+    expect(t).toContain('4')
+    expect(t).toContain('No artist')
+  })
+
+  it('shows an en-dash placeholder for a flag count before facets load', () => {
+    const pinia = freshPinia()
+    stubFetchOk(FACETS)
+    useGalleryStore().facets = null  // not loaded yet
+    const w = mountComponent(GalleryFacetPanel, { pinia })
+    expect(w.text()).toContain('Untagged')
+    expect(w.text()).toContain('–')
+  })
+})