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/ChannelDiscovery.tsx

@@ -23,7 +23,7 @@ import Pager from "./ui/Pager";
 import { PageToolbar } from "./ui/PageShell";
 import { useConfirm } from "./confirmContext";
 import { useChannelLayout } from "./channelsContext";
-import { FocusSafeButton } from "./ui/FocusSafeButton";
+import { Button } from "./ui/Button";
 
 // The Channel manager's "Discovery" tab: channels that turn up in the user's playlists but
 // that they don't subscribe to. Subscribing here only creates the subscription + enriches
@@ -108,16 +108,16 @@ export default function ChannelDiscovery({
             canWrite ? t("channels.discovery.subscribeHint") : t("channels.discovery.needWriteHint")
           }
         >
-          <FocusSafeButton
-            // Busy blocks the click, not the focus (FocusSafeButton).
+          <Button
+            variant="glass"
+            size="xs"
+            icon={<UserPlus />}
             onClick={() => onSubscribe(c)}
             disabled={!canWrite}
-            busy={subscribe.isPending}
-            className="glass-card glass-hover inline-flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-xs disabled:opacity-50 aria-disabled:opacity-50 transition"
+            loading={subscribe.isPending}
           >
-            <UserPlus className="w-3.5 h-3.5" />
             {t("channels.discovery.subscribe")}
-          </FocusSafeButton>
+          </Button>
         </Tooltip>
       ),
     },