refactor(ui): R10 S4 — unify selected-paint into .pill/.is-current/.chip recipes

The ~14 hand-rolled "this is the selected/current/active one" idioms each
repeated the same `sel ? "bg-accent text-accent-fg border-accent" : "…"` inline
ternary, which had drifted. Three attribute-driven recipes in @layer components
now own that paint, keyed on the ARIA the control already carries: .pill
(aria-pressed segments/toggles), .is-current (aria-current nav/list items), and
.chip / .chip-solid (tag & filter tokens). The JSX just toggles the ARIA; the CSS
paints, so one control's selected look can't diverge from the next's. No
focus-visible in the recipes — the @layer base rule already rings every button/a.

Migrated 15 sites (Feed show/content, Channels tag filter, Sidebar or-and +
scope, PlexSidebar match, Tabs, SettingsPanel tab-rail, PlexPlaylistView layout
[soft], NavSidebar nav + settings, ActiveFilterChips, PlexSidebar RemovableChip,
ChannelTags, ChannelPicker). Deliberately left: PlexSidebar facet Chip and
PlexPlaylistView already covered; SavedViews row (paint split across two
elements), the scheme swatch (border-only on a coloured swatch), and
ChannelPicker's option rows (text-only selection) — noted for a follow-up.

frontend/src/components/Channels.tsx

@@ -667,11 +667,7 @@ export default function Channels() {
                         setTagFilter((f) => (active ? f.filter((x) => x !== tg.id) : [...f, tg.id]))
                       }
                       aria-pressed={active}
-                      className={`text-xs px-2 py-1 rounded-full border transition ${
-                        active
-                          ? "bg-accent text-accent-fg border-accent"
-                          : "bg-card border-border text-muted hover:text-fg hover:border-accent"
-                      }`}
+                      className="pill text-xs px-2 py-1 rounded-full border"
                     >
                       {tg.name}
                     </button>