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.

frontend/src/lib/urlState.dom.test.ts

@@ -121,4 +121,23 @@ describe("filterParamUpdates", () => {
     expect(back.scope).toBe("all");
     expect(back.includeShorts).toBe(true);
   });
+
+  it("round-trips per-category tag modes (only 'and' is emitted)", () => {
+    const f: FeedFilters = {
+      ...DEFAULTS,
+      tags: [1, 2],
+      tagModes: { language: "and", topic: "and", other: "or" },
+    };
+    const params = filtersToParams(f);
+    expect(params.get("tagmode_language")).toBe("and");
+    expect(params.get("tagmode_topic")).toBe("and");
+    expect(params.has("tagmode_other")).toBe(false); // 'or' is the default, not emitted
+    const back = paramsToFilters(params, DEFAULTS);
+    expect(back.tagModes).toEqual({ language: "and", topic: "and", other: "or" });
+  });
+
+  it("migrates a legacy global ?tagmode=and into the topic slot only", () => {
+    const back = paramsToFilters(new URLSearchParams("tagmode=and"), DEFAULTS);
+    expect(back.tagModes).toEqual({ language: "or", topic: "and", other: "or" });
+  });
 });