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/channelCells.tsx

@@ -4,7 +4,7 @@ import type { ManagedChannel } from "../lib/api";
 import { formatNumber } from "../lib/format";
 import Tooltip from "./ui/Tooltip";
 import { isFullySynced } from "./channelSync";
-import { FocusSafeButton } from "./ui/FocusSafeButton";
+import { IconButton } from "./ui/IconButton";
 
 // The Channels-manager cells that are components in their own right (they read i18n), split out of
 // channelColumns so that file holds only the column factory and its predicates (Fast Refresh).
@@ -67,39 +67,35 @@ export function ActionsCell({
   return (
     <div className="inline-flex items-center gap-2">
       <Tooltip hint={backfillHint}>
-        <FocusSafeButton
+        <IconButton
           onClick={!enabled ? undefined : isAdmin ? onReset : onDeep}
           disabled={!enabled}
-          className={`shrink-0 transition ${
-            requested
-              ? "text-accent"
-              : enabled
-                ? "text-muted hover:text-fg"
-                : "text-muted/40 cursor-default"
-          }`}
-          aria-label={isAdmin ? t("channels.row.resetBackfill") : t("channels.row.backfillThis")}
+          variant={requested ? "ghost-accent" : "ghost"}
+          label={isAdmin ? t("channels.row.resetBackfill") : t("channels.row.backfillThis")}
+          noTitle
         >
-          <History className="w-4 h-4" />
-        </FocusSafeButton>
+          <History />
+        </IconButton>
       </Tooltip>
       <Tooltip hint={c.hidden ? t("channels.row.hiddenHint") : t("channels.row.hideHint")}>
-        <button
+        <IconButton
           onClick={onHide}
-          className="text-muted hover:text-fg shrink-0"
-          aria-label={c.hidden ? t("channels.row.unhide") : t("channels.row.hideFromFeed")}
+          label={c.hidden ? t("channels.row.unhide") : t("channels.row.hideFromFeed")}
+          noTitle
         >
-          {c.hidden ? <EyeOff className="w-4 h-4" /> : <Eye className="w-4 h-4" />}
-        </button>
+          {c.hidden ? <EyeOff /> : <Eye />}
+        </IconButton>
       </Tooltip>
       {canWrite && (
         <Tooltip hint={t("channels.row.unsubscribeHint")}>
-          <button
+          <IconButton
+            variant="danger"
             onClick={onUnsubscribe}
-            className="text-muted hover:text-danger shrink-0"
-            aria-label={t("channels.row.unsubscribeOnYoutube")}
+            label={t("channels.row.unsubscribeOnYoutube")}
+            noTitle
           >
-            <UserMinus className="w-4 h-4" />
-          </button>
+            <UserMinus />
+          </IconButton>
         </Tooltip>
       )}
     </div>