feat(feed): compress the toolbar chip groups into labeled FilterSelect dropdowns

Adds a reusable FilterSelect: a "Label: value ▾" pill that opens a listbox (a checkmark
row for single-select, a checkbox row for multi), on the AnchoredPopover platform. It
replaces the two feed-toolbar chip walls:
  - Watch state (single-select, default All) — picking a row replaces + closes.
  - Content types (multi-select, default Normal; requireOne keeps at least one on) — rows
    toggle and the panel stays open.
The trigger summarises multi picks as "First +N". The component is source-agnostic, meant
for reuse anywhere a small known option set currently renders as a chip group.

FilterSelect takes a `testId` → the trigger gets it, each option gets `${testId}-${value}`,
so the feed/channel E2Es keep stable selectors. They now open the dropdown and assert the
option's aria-selected instead of the old inline aria-pressed chips (new expectContentType/
toggleContentType helpers).

frontend/e2e/feed.spec.ts

@@ -12,11 +12,14 @@ test.describe("feed", () => {
     await expect(count).toHaveText(/\d/);
     const baseline = await count.innerText();
 
-    // Clean baseline: default filters, so no chips and "unwatched" is the active show.
+    // Clean baseline: default filters, so no chips and "unwatched" is the active show (the Watch-state
+    // pill summarises the current pick).
     await expect(page.getByTestId("active-filter-chip")).toHaveCount(0);
-    await expect(page.getByTestId("feed-show-unwatched")).toHaveAttribute("aria-pressed", "true");
+    await expect(page.getByTestId("feed-show")).toContainText("Unwatched");
 
-    // Narrow to "watched" — the seed guarantees exactly one, a real deterministic change.
+    // Narrow to "watched" — open the Watch-state dropdown and pick it (single-select closes on pick).
+    // The seed guarantees exactly one, a real deterministic change.
+    await page.getByTestId("feed-show").click();
     await page.getByTestId("feed-show-watched").click();
     await expect(page.getByTestId("active-filter-chip")).toHaveCount(1);
     await expect(count).not.toHaveText(baseline);
@@ -24,7 +27,7 @@ test.describe("feed", () => {
     // Remove the chip → default show restored, chips gone.
     await page.getByTestId("active-filter-chip").click();
     await expect(page.getByTestId("active-filter-chip")).toHaveCount(0);
-    await expect(page.getByTestId("feed-show-unwatched")).toHaveAttribute("aria-pressed", "true");
+    await expect(page.getByTestId("feed-show")).toContainText("Unwatched");
   });
 
   test("scrolling virtualizes in fresh cards", async ({ page }) => {
@@ -73,6 +76,7 @@ test.describe("feed", () => {
     expect(await page.getByRole("main").evaluate((el) => el.scrollTop)).toBeGreaterThan(500);
 
     // Switch the show filter → a different result set → back to the top.
+    await page.getByTestId("feed-show").click();
     await page.getByTestId("feed-show-all").click();
     await expect.poll(() => page.getByRole("main").evaluate((el) => el.scrollTop)).toBeLessThan(50);
   });