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/index.css

@@ -96,6 +96,16 @@
     background-color: color-mix(in srgb, var(--danger-fg) 24%, var(--danger));
   }
 }
+@utility state-warn {
+  background-color: var(--warn);
+  color: var(--warn-fg);
+  &:hover:not(:disabled, [aria-disabled="true"]) {
+    background-color: color-mix(in srgb, var(--warn-fg) 14%, var(--warn));
+  }
+  &:active:not(:disabled, [aria-disabled="true"]) {
+    background-color: color-mix(in srgb, var(--warn-fg) 24%, var(--warn));
+  }
+}
 /* A4: a control's ≥ 3:1 edge. */
 @utility edge-strong {
   border-color: var(--edge-strong);
@@ -618,30 +628,6 @@ body {
     color: var(--accent-fg);
     border-color: var(--accent);
   }
-  /* .btn-{tone} — solid ACTION-button paint (the "primary/danger/warn action", distinct from the
-     selection recipes above which are "selected among options"). Each sets its tone fill + contrasting
-     fg; all share ONE standard hover-dim. Like .chip-solid this owns paint (+ that single hover
-     behaviour), NOT geometry — a site keeps its own padding/radius/font/gap/disabled:opacity/shadow.
-     Migrated sites MUST drop `bg-{tone} text-{tone}-fg hover:opacity-90` so these @layer-components
-     rules aren't beaten by those utilities. No focus-visible — @layer base already rings every button. */
-  .btn-primary {
-    background-color: var(--accent);
-    color: var(--accent-fg);
-  }
-  .btn-danger {
-    background-color: var(--danger);
-    color: var(--danger-fg);
-  }
-  .btn-warn {
-    background-color: var(--warn);
-    color: var(--warn-fg);
-  }
-  .btn-primary:hover:not(:disabled):not([aria-disabled="true"]),
-  .btn-danger:hover:not(:disabled):not([aria-disabled="true"]),
-  .btn-warn:hover:not(:disabled):not([aria-disabled="true"]) {
-    opacity: 0.9;
-  }
-
   /* ===== E10 S1c · the minimum-target half of the tablet comfort contract (docs/DESIGN.md) =====
      A 32px icon button is fine with a mouse (a precise pointer plus hover to confirm the aim) and
      not fine with a thumb. So the minimum grows ONLY where there is no precise pointer, which on