feat(gallery,tags): clear active filters

Two gaps where a filter couldn't be removed:

- Gallery: a tag_id filter (from clicking a tag) had no indicator or clear
  control — only post_id did (PostInfoHeader). Add an "Tag: <name> ✕" chip
  that clears the filter by dropping tag_id from the URL. New lightweight
  GET /api/tags/<id> resolves the name; the store fetches it on filter set.
- Tags view: the kind chip-group used mandatory="false" — a STRING ("false"
  is truthy in JS), which made the group mandatory so the active kind chip
  couldn't be deselected. Fixed to :mandatory="false" so the filter clears.

Tests: GET /tags/<id> shape + 404; gallery store resolves filterTagName.

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

frontend/test/gallery.spec.js

@@ -50,6 +50,19 @@ describe('gallery store: tag/post filter exclusivity', () => {
     expect(scrollCall).not.toContain('tag_id=')
   })
 
+  it('setTagFilter resolves the tag name for the filter chip', async () => {
+    const s = useGalleryStore()
+    stubFetch((url) => {
+      if (url.includes('/api/tags/7')) {
+        return { status: 200, body: { id: 7, name: 'Asuka', kind: 'character' } }
+      }
+      return { status: 200, body: EMPTY }  // scroll + timeline
+    })
+    s.setTagFilter(7)
+    expect(s.filter.tag_id).toBe(7)
+    await vi.waitFor(() => expect(s.filterTagName).toBe('Asuka'))
+  })
+
   it('loadInitial issues exactly one scroll request at the initial limit', async () => {
     const s = useGalleryStore()
     const urls = []