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/Feed.tsx

@@ -557,11 +557,7 @@ export default function Feed({
             data-testid={`feed-show-${id}`}
             onClick={() => setFilters({ ...filters, show: id })}
             aria-pressed={filters.show === id}
-            className={`text-xs px-3 py-1.5 rounded-full border transition ${
-              filters.show === id
-                ? "bg-accent text-accent-fg border-accent"
-                : "border-border text-muted hover:text-fg hover:border-accent"
-            }`}
+            className="pill text-xs px-3 py-1.5 rounded-full border"
           >
             {t("sidebar.show." + id)}
           </button>
@@ -575,11 +571,7 @@ export default function Feed({
               data-testid={`feed-content-${c.key}`}
               onClick={() => setFilters({ ...filters, [c.key]: !on })}
               aria-pressed={on}
-              className={`text-xs px-3 py-1.5 rounded-full border transition ${
-                on
-                  ? "bg-accent text-accent-fg border-accent"
-                  : "border-border text-muted hover:text-fg hover:border-accent"
-              }`}
+              className="pill text-xs px-3 py-1.5 rounded-full border"
             >
               {t(c.label)}
             </button>