feat(design): every button on its family, the chip primitive, the lint rule (E28 S3b2)

The second half of the button sweep: the remaining hand-rolled buttons in
feature components, the design lab and the components/ui composites now
stand on Button, IconButton, Pressable, Chip or DragHandle.

- Chip + DismissibleChip (filter / token / tag / suggest / ghost, 24px
  floor, 44 on touch) replace five chip geometries; FilterChipGroup
  renders Chips; overflow lanes measure a mirror painted by chipClass.
- The Plex control row is IconButton's player rung (36, 20px glyph,
  round, overlay; 44 on touch); the YT modal bar is on the sm rung with
  pressed cycle toggles and names that survive a folded label.
- New variants: IconButton ghost-accent, danger-soft; Button warn on a
  new state-warn utility. The .btn-{tone} recipes and btnCls are gone.
- Fixed on the way: membership rows (collection editor, Plex playlist
  add) announce checked/mixed; a one-letter tag chip was 21px wide;
  descriptionLinks moved out of lib/ (it renders components).
- local/button-primitive: a raw <button>/<FocusSafeButton> outside the
  control-primitive files is a lint error (contract-tested, seen red).

frontend/src/components/ExcludedChannelList.tsx

@@ -2,6 +2,7 @@ import { type ReactNode } from "react";
 import ChannelLink from "./ChannelLink";
 import NavGrid from "./ui/NavGrid";
 import { navItemAttrs } from "../lib/listNav";
+import { Button } from "./ui/Button";
 
 // The presentation shared by the channel manager's "excluded channel" tabs — Hidden and Blocked.
 // A responsive grid of proper channel cards (avatar + name + open-on-YouTube, via ChannelLink) each
@@ -58,15 +59,9 @@ export default function ExcludedChannelList({
                 onView={onView ? () => onView(c) : undefined}
               />
             </div>
-            <button
-              onClick={() => onAction(c.id)}
-              title={actionLabel}
-              aria-label={actionLabel}
-              className="shrink-0 inline-flex items-center gap-1 rounded-full border border-border px-2 py-1 text-xs text-muted transition hover:border-accent hover:text-accent"
-            >
-              {actionIcon}
+            <Button variant="outline" size="xs" icon={actionIcon} onClick={() => onAction(c.id)}>
               {actionLabel}
-            </button>
+            </Button>
           </div>
         ))}
       </NavGrid>