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/SidePanel.tsx

@@ -4,6 +4,7 @@ import { Check, Pencil, Pin, PinOff, RotateCcw } from "lucide-react";
 import { useScrollFade } from "../../lib/useScrollFade";
 import { useHoverFocusWithin } from "../../lib/useHoverFocusWithin";
 import { IconButton } from "./IconButton";
+import { Pressable } from "./Pressable";
 
 // The shared floating side panel — one shell for the Feed / Plex / Playlists rails so they read as
 // one system, and the same peek-open model as the nav rail: unpinned it sits as a small tab docked
@@ -72,7 +73,7 @@ export default function SidePanel({
           nowhere. It only goes transparent: the card's rounded corner cuts away its own pixels, and
           this square-cornered tab sits right under that arc, so it would otherwise bleed through.
           `opacity-0` (not `invisible`/`hidden`) keeps it hoverable and focusable. */}
-      <button
+      <Pressable
         {...peek.hoverHandlers}
         onClick={onToggleCollapse}
         aria-label={pinned ? t("sidebar.unpin") : t("sidebar.pin")}
@@ -92,7 +93,7 @@ export default function SidePanel({
         >
           {title}
         </span>
-      </button>
+      </Pressable>
 
       {expanded && (
         <aside
@@ -113,14 +114,14 @@ export default function SidePanel({
             </IconButton>
             {iconExpanded ?? <span className="shrink-0 flex text-accent">{icon}</span>}
             {onTitleClick ? (
-              <button
+              <Pressable
                 onClick={onTitleClick}
                 title={titleActionLabel}
                 aria-label={titleActionLabel}
-                className="flex-1 min-w-0 truncate text-left text-sm font-bold uppercase tracking-[0.12em] transition hover:text-accent"
+                className="flex-1 min-w-0 truncate text-sm font-bold uppercase tracking-[0.12em] transition hover:text-accent"
               >
                 {title}
-              </button>
+              </Pressable>
             ) : (
               <span className="flex-1 min-w-0 truncate text-sm font-bold uppercase tracking-[0.12em]">
                 {title}
@@ -145,15 +146,13 @@ export default function SidePanel({
               </IconButton>
             )}
             {onToggleEditing && (
-              <button
+              <IconButton
+                variant={editing ? "ghost-accent" : "ghost"}
                 onClick={onToggleEditing}
-                title={editing ? t("sidebar.done") : t("sidebar.customize")}
-                className={`shrink-0 p-1.5 rounded-lg transition hover:bg-card ${
-                  editing ? "text-accent" : "text-muted hover:text-fg"
-                }`}
+                label={editing ? t("sidebar.done") : t("sidebar.customize")}
               >
-                {editing ? <Check className="w-4 h-4" /> : <Pencil className="w-4 h-4" />}
-              </button>
+                {editing ? <Check /> : <Pencil />}
+              </IconButton>
             )}
           </div>