feat(feed): per-category any/all tag match, in each combobox's popover footer

The tag 'Match: Any/All' toggle was a single GLOBAL setting that only ever
affected topics, shown as its own row under the Topic widget. Make it
per-category — language, topic, and 'your tags' each get their own independent
any/all — and move it into each filter combobox's popover footer (like
SortControl's direction segment), reclaiming the standalone Match row.

Backend: the feed query already groups selected tags by category; replace the
single tag_mode with per-category tag_mode_language/topic/other params and apply
the conjunctive (HAVING all) branch per category. Mirrored across filtered_query,
feed_summary.passes(), and the /facets drill-down exclusion (kept in lockstep;
equivalence test updated to tag_mode_topic). No schema change.

Frontend: FeedFilters.tagMode -> tagModes {language,topic,other} (pure leaf
lib/api/tagModes with a coerceTagModes migration for the legacy global value);
serialized to the API and the shareable URL per category (legacy ?tagmode=and
still resolves into the topic slot); saved-view blobs migrate on read. Combobox
gains an optional matchMode/onMatchModeChange footer segment, shown once 2+
values are picked; the three tag comboboxes wire it to their category.

Verified: backend feed tests green (12, incl. the count/facets equivalence net);
live, the footer appears at 2+ selections, toggles aria-pressed, serializes
tagmode_topic=and to the URL, and the request carries tag_mode_topic=and (200).
374 vitest, tsc, ruff, eslint (0 err).

backend/tests/test_feed_views.py

@@ -127,8 +127,8 @@ _SUMMARY_CASES = [
     "show=all&tags={L1}",
     "show=all&tags={T1}",
     "show=all&tags={L1}&tags={T1}",
-    "show=all&tags={T1}&tags={T2}&tag_mode=or",
-    "show=all&tags={T1}&tags={T2}&tag_mode=and",
+    "show=all&tags={T1}&tags={T2}&tag_mode_topic=or",
+    "show=all&tags={T1}&tags={T2}&tag_mode_topic=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",