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/PlaylistsRail.tsx

@@ -23,6 +23,8 @@ import { useMe } from "../lib/useMe";
 import { playlistName } from "../lib/playlistName";
 import { usePlaylistActions } from "./usePlaylistActions";
 import PlaylistLimitsHint from "./PlaylistLimitsHint";
+import { IconButton } from "./ui/IconButton";
+import { Pressable } from "./ui/Pressable";
 
 type PlSort = {
   key: "custom" | "name" | "count" | "duration" | "recent";
@@ -255,14 +257,14 @@ export default function PlaylistsRail({
                 next && setPlSort({ ...plSort, key: next.key as PlSort["key"], dir: next.dir })
               }
               trailing={
-                <button
+                <IconButton
+                  variant="outline"
                   onClick={() => setPlSort({ ...plSort, dirtyFirst: !plSort.dirtyFirst })}
-                  title={t("playlists.dirtyFirst")}
-                  aria-pressed={plSort.dirtyFirst}
-                  className="pill shrink-0 rounded-lg border p-1.5"
+                  label={t("playlists.dirtyFirst")}
+                  pressed={plSort.dirtyFirst}
                 >
-                  <Pin className="h-4 w-4" />
-                </button>
+                  <Pin />
+                </IconButton>
               }
             />
           )}
@@ -292,13 +294,12 @@ export default function PlaylistsRail({
         <>
           {playlists.length === 0 &&
             (listQuery.isError && !listQuery.data ? (
-              <button
-                type="button"
+              <Pressable
                 onClick={() => listQuery.refetch()}
-                className="text-xs text-danger px-1 py-1 text-left hover:underline"
+                className="text-xs text-danger px-1 py-1 hover:underline"
               >
                 {t("common.loadError")} — {t("common.retry")}
-              </button>
+              </Pressable>
             ) : listQuery.isLoading ? (
               <div className="text-xs text-muted px-1 py-1">{t("common.loading")}</div>
             ) : (
@@ -342,7 +343,7 @@ export default function PlaylistsRail({
                   />
                 </div>
               ) : (
-                <button
+                <Pressable
                   key={pl.id}
                   ref={(el) => {
                     if (pl.id === selectedId) selectedRef.current = el;
@@ -354,10 +355,9 @@ export default function PlaylistsRail({
                   }}
                   onClick={() => setSelectedId(pl.id)}
                   {...railMenu.triggerProps(pl)}
-                  className={`${ROW_BASE} text-left ${
-                    pl.id === selectedId
-                      ? "bg-accent/15 border-accent"
-                      : "border-transparent hover:bg-card/60"
+                  layer={pl.id !== selectedId}
+                  className={`${ROW_BASE} ${
+                    pl.id === selectedId ? "bg-accent/15 border-accent" : "border-transparent"
                   }`}
                 >
                   <div className="w-8.5 h-8.5 rounded-md bg-card overflow-hidden shrink-0">
@@ -380,7 +380,7 @@ export default function PlaylistsRail({
                         ` · ${formatRuntime(pl.total_duration_seconds)}`}
                     </div>
                   </div>
-                </button>
+                </Pressable>
               ),
             )}
           </div>