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>

backend/app/api/tags.py

@@ -194,6 +194,24 @@ async def remove_tag_from_image(image_id: int, tag_id: int):
     return "", 204
 
 
+@tags_bp.route("/tags/<int:tag_id>", methods=["GET"])
+async def get_tag(tag_id: int):
+    """Resolve a single tag (used by the gallery to label its active
+    tag-filter chip)."""
+    async with get_session() as session:
+        tag = await session.get(Tag, tag_id)
+        if tag is None:
+            return jsonify({"error": "tag not found"}), 404
+        return jsonify(
+            {
+                "id": tag.id,
+                "name": tag.name,
+                "kind": tag.kind.value,
+                "fandom_id": tag.fandom_id,
+            }
+        )
+
+
 @tags_bp.route("/tags/<int:tag_id>", methods=["PATCH"])
 async def update_tag(tag_id: int):
     """Rename and/or re-fandom a tag. Body may carry `name` and/or