fix(design): the S3g conformance sweep — primitives at their rung (E28 S3g)

The probe re-run over the 15 pages at 1920x1080 found every button, icon
button, chip, field, tab, segment and menu row drawn by its primitive
except the S3h Plex picture, a div with the button role. It is a
Pressable now, and the button-primitive lint rule refuses an element
with role="button" outside the control primitives.

- SegmentedToggle defaults to the toolbar rung (28, text-xs): the old
  small default had put the pager's page sizes and the YouTube result
  count on 10px type beside 28px buttons. The Plex match toggle asks
  for the small rung explicitly.
- The language badge on the rail uses text-3xs (was an 8px one-off);
  the Plex playlist's watched mark and the Home hero's length use
  MEDIA_BADGE instead of hand-copied classes.
- A media picture shows the pointer whether it is a link or a button.
- DESIGN.md: a Conformance chapter (the four-step check before a UI
  change is done, with the S3g baseline), the segmented rung, two log
  rows. CLAUDE.md points at the check.

frontend/src/components/plexCards.dom.test.tsx

@@ -270,7 +270,10 @@ describe("Plex cards in a keyboard-walkable list", () => {
     const root = container.firstElementChild as HTMLElement;
     expect(root.getAttribute("data-nav-index")).toBe("3");
     const open = root.querySelector<HTMLElement>("[data-nav-open]")!;
-    expect(open.getAttribute("role")).toBe("button");
+    // A real button (ui/Pressable), not a div wearing its role: Enter/Space and the touch target
+    // come from the primitive (S3g found the S3h picture hand-built).
+    expect(open.tagName).toBe("BUTTON");
+    expect(open.getAttribute("type")).toBe("button");
     fireEvent.click(open);
     expect(onOpen).toHaveBeenCalledTimes(1);
   });