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>

tests/test_api_tag_merge.py

@@ -11,6 +11,19 @@ async def _mk(client, name, kind, fandom_id=None):
     return (await r.get_json())["id"]
 
 
+@pytest.mark.asyncio
+async def test_get_tag_returns_shape_and_404(client):
+    tid = await _mk(client, "Lookup", "general")
+    resp = await client.get(f"/api/tags/{tid}")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["id"] == tid
+    assert body["name"] == "Lookup"
+    assert body["kind"] == "general"
+    resp = await client.get("/api/tags/99999999")
+    assert resp.status_code == 404
+
+
 @pytest.mark.asyncio
 async def test_patch_sets_and_clears_character_fandom(client):
     fandom = await _mk(client, "Bleach", "fandom")