test(feed): cover the filter-bar gaps deep-review flagged

- test_feed_views: extend _seed_tagged so a channel carries 2+ tags in a
  non-topic category, and add language-AND / other-AND equivalence cases — the
  per-category any/all generalization was only exercised for topic.
- tagModes.test: coerceTagModes migration (legacy global tagMode -> topic),
  per-category preservation, invalid-value coercion, defaults.
- urlState.dom.test: per-category tag-mode URL round-trip + legacy ?tagmode=and
  migration into the topic slot.
- Combobox.dom.test: the any/all match footer (hidden below 2 selections, reports
  a mode change at 2+).
- chipOverflow.test: the all/some/count fit boundaries and the +N reservation.

backend/tests/test_feed_views.py

@@ -96,6 +96,7 @@ def _seed_tagged(db, make_user):
         "T1": Tag(user_id=None, name="Tech", category="topic"),
         "T2": Tag(user_id=None, name="Gaming", category="topic"),
         "O1": Tag(user_id=None, name="Favourite", category="other"),
+        "O2": Tag(user_id=None, name="Docs", category="other"),
         # A USER-owned tag (+ a user-owned ChannelTag link below) so the visibility branch
         # `ChannelTag.user_id == user.id` is exercised, not only the system `user_id IS NULL` one.
         "U1": Tag(user_id=user.id, name="MyPin", category="other"),
@@ -104,8 +105,15 @@ def _seed_tagged(db, make_user):
     chans = {c: Channel(id=c, title=c, subscriber_count=10) for c in ("chA", "chB", "chC", "chD")}
     db.add_all(chans.values())
     db.flush()
-    # channel -> (system) tag keys
-    links = {"chA": ["L1", "T1"], "chB": ["L1", "T2"], "chC": ["L2", "T1", "O1"], "chD": []}
+    # channel -> (system) tag keys. chA carries two languages (L1+L2) and chC two "other" tags
+    # (O1+O2) so the per-category any/all match narrows non-trivially for language and other, not
+    # only topic (chA/chB share topic tags T1/T2).
+    links = {
+        "chA": ["L1", "L2", "T1"],
+        "chB": ["L1", "T2"],
+        "chC": ["L2", "T1", "O1", "O2"],
+        "chD": [],
+    }
     for cid, tkeys in links.items():
         db.add(Subscription(user_id=user.id, channel_id=cid, priority=0, hidden=False))
         for tk in tkeys:
@@ -129,6 +137,10 @@ _SUMMARY_CASES = [
     "show=all&tags={L1}&tags={T1}",
     "show=all&tags={T1}&tags={T2}&tag_mode_topic=or",
     "show=all&tags={T1}&tags={T2}&tag_mode_topic=and",
+    # Per-category any/all is NOT topic-only — language and other must drill down identically too.
+    "show=all&tags={L1}&tags={L2}&tag_mode_language=or",
+    "show=all&tags={L1}&tags={L2}&tag_mode_language=and",
+    "show=all&tags={O1}&tags={O2}&tag_mode_other=and",
     "show=all&tags={U1}",  # user-owned tag (visibility == user.id branch)
     "show=all&tags={U1}&tags={L1}",  # user tag AND a system tag, cross-category
     "show=all&q=clip",