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/LanguageSwitcher.tsx

@@ -3,6 +3,7 @@ import { Check, Globe } from "lucide-react";
 import { LANGUAGES, type LangCode } from "../i18n";
 import { PopoverItem, PopoverSurface } from "./ui/AnchoredPopover";
 import { useAnchoredPopover } from "./ui/anchoredPopoverHooks";
+import { Pressable } from "./ui/Pressable";
 
 // Compact language picker (globe + current code). Presentational: the parent decides what
 // changing the language does (set it; persist server-side when signed in).
@@ -42,15 +43,16 @@ export default function LanguageSwitcher({
 
   return (
     <>
-      <button
+      <Pressable
         ref={pop.refs.setReference}
         {...pop.getReferenceProps()}
         title={current.label}
         aria-label={current.label}
+        layer
         className={
           rail
-            ? "relative p-2 rounded-lg text-muted hover:text-fg hover:bg-card transition"
-            : "flex items-center gap-1.5 px-2.5 py-1.5 rounded-lg text-sm text-muted hover:text-fg hover:bg-card transition"
+            ? "relative p-2 rounded-lg text-muted hover:text-fg transition"
+            : "flex h-7 items-center gap-1.5 px-2.5 rounded-lg text-sm text-muted hover:text-fg transition"
         }
       >
         <Globe className={rail ? "w-5 h-5" : "w-4 h-4"} />
@@ -63,7 +65,7 @@ export default function LanguageSwitcher({
         ) : (
           <span className="text-xs font-semibold uppercase">{current.code}</span>
         )}
-      </button>
+      </Pressable>
 
       <PopoverSurface
         popover={pop}