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/Channels.tsx

@@ -524,12 +524,13 @@ export default function Channels() {
               </Tooltip>
               <TagFilterChips chips={tagFilterChips} className="flex-1" />
               {tagFilter.length > 0 && (
-                <button
+                <Button
+                  variant="link-muted"
                   onClick={() => setFilters({ ...filters, tags: [] })}
-                  className="shrink-0 text-xs px-2 py-1 text-muted hover:text-accent transition"
+                  className="text-xs px-2"
                 >
                   {t("datatable.clear")}
-                </button>
+                </Button>
               )}
               {/* Suggested-tags affordance (E8·S4): a sparkle count of the visible suggestions, with a
                   one-click bulk-accept for the high-confidence ones. The per-row ghost chips handle the