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/src/stores/gallery.js

@@ -22,6 +22,9 @@ export const useGalleryStore = defineStore('gallery', () => {
   const loading = ref(false)
   const error = ref(null)
   const filter = ref({ tag_id: null, post_id: null })
+  // Display name for the active tag filter, so the gallery can label a
+  // "Tag: X ✕" chip instead of leaving the filter invisible/unclearable.
+  const filterTagName = ref(null)
 
   const timelineBuckets = ref([])
   const timelineLoading = ref(false)
@@ -94,10 +97,20 @@ export const useGalleryStore = defineStore('gallery', () => {
   function setTagFilter(tagId) {
     filter.value.tag_id = tagId
     filter.value.post_id = null
+    filterTagName.value = null
+    if (tagId) _resolveTagName(tagId)  // fire-and-forget; chip label only
     loadInitial()
     loadTimeline()
   }
 
+  async function _resolveTagName(tagId) {
+    try {
+      filterTagName.value = (await api.get(`/api/tags/${tagId}`)).name
+    } catch {
+      // Leave null — the chip falls back to "#<id>".
+    }
+  }
+
   function setPostFilter(postId) {
     filter.value.post_id = postId
     filter.value.tag_id = null
@@ -110,7 +123,7 @@ export const useGalleryStore = defineStore('gallery', () => {
 
   return {
     images, dateGroups, hasMore, isEmpty, loading, error,
-    filter, timelineBuckets, timelineLoading,
+    filter, filterTagName, timelineBuckets, timelineLoading,
     loadInitial, loadMore, loadTimeline, jumpTo, setTagFilter, setPostFilter
   }
 })