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/ui/Toaster.tsx

@@ -6,6 +6,8 @@ import { createStore } from "../../lib/store";
 import { useToastDock } from "../../lib/toastDock";
 import { useOverlayOrder } from "./overlayOrder";
 import { LEVEL_STYLE } from "./toastLevels";
+import { Button } from "./Button";
+import { IconButton } from "./IconButton";
 
 // Overlay hosts. The app's own Toaster sits in the content column, and that column is a stacking
 // context (its `@container` implies layout containment) — so anything portalled to <body> at the
@@ -170,24 +172,25 @@ export default function Toaster({
                   )}
                 </div>
                 {toast.action && (
-                  <button
+                  <Button
+                    variant="link"
                     onClick={() => {
                       toast.action!.onClick();
                       dismiss(toast.id);
                     }}
-                    className="mt-1 text-accent text-sm font-semibold hover:underline"
+                    className="mt-1 text-sm"
                   >
                     {toast.action.label}
-                  </button>
+                  </Button>
                 )}
               </div>
-              <button
+              <IconButton
                 onClick={() => dismiss(toast.id)}
-                className="shrink-0 text-muted hover:text-fg"
-                title={t("notifications.dismiss")}
+                label={t("notifications.dismiss")}
+                className="-m-1.5"
               >
-                <X className="w-4 h-4" />
-              </button>
+                <X />
+              </IconButton>
               {toast.duration && (
                 // Keyed by `ts`: a coalesced repeat re-arms the dismiss timer, so its bar restarts too.
                 <CountdownBar key={toast.ts} since={toast.ts} duration={toast.duration} bar={bar} />