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/components/Combobox.dom.test.tsx

@@ -205,6 +205,31 @@ describe("Combobox", () => {
     expect(onChange).toHaveBeenCalledWith(["id1"]); // single-select replaces
   });
 
+  it("shows the any/all match footer only at 2+ selections and reports a mode change", () => {
+    // The per-category any/all toggle: meaningless below 2 selections (any === all), so it appears
+    // only at >= 2, and clicking 'all' reports the mode change out.
+    const onMatchModeChange = vi.fn();
+    const props = {
+      label: "Topic",
+      options: OPTIONS,
+      onChange: vi.fn(),
+      matchMode: "or" as const,
+      onMatchModeChange,
+    };
+    // One selection: no footer.
+    const { rerender } = wrap(<Combobox {...props} selected={["Drama"]} />);
+    fireEvent.focus(screen.getByRole("combobox"));
+    expect(screen.queryByText("sidebar.all")).toBeNull();
+
+    // Two selections: footer appears with an any/all segment.
+    rerender(<FloatingTree>{<Combobox {...props} selected={["Drama", "Action"]} />}</FloatingTree>);
+    fireEvent.focus(screen.getByRole("combobox"));
+    const all = screen.getByText("sidebar.all");
+    expect(all).toBeTruthy();
+    fireEvent.click(all);
+    expect(onMatchModeChange).toHaveBeenCalledWith("and");
+  });
+
   it("single-select: picking replaces the value and closes the list", () => {
     const onChange = vi.fn();
     wrap(