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/SettingsPanel.tsx

@@ -69,10 +69,9 @@ export default function SettingsPanel() {
                   key={tabItem.id}
                   data-testid={`settings-tab-${tabItem.id}`}
                   onClick={() => setTab(tabItem.id)}
-                  className={`flex items-center gap-2 px-3 py-2 rounded-lg text-sm text-left transition ${
-                    active
-                      ? "bg-accent text-accent-fg font-medium shadow-md shadow-accent/20"
-                      : "text-muted hover:text-fg hover:bg-card/60"
+                  aria-pressed={active}
+                  className={`pill flex items-center gap-2 px-3 py-2 rounded-lg text-sm text-left ${
+                    active ? "font-medium shadow-md shadow-accent/20" : "hover:bg-card/60"
                   }`}
                 >
                   <tabItem.icon className="w-4 h-4 shrink-0" />