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/ChannelPicker.tsx

@@ -52,7 +52,7 @@ export default function ChannelPicker({
               key={id}
               onClick={() => toggle(id)}
               title={t("sidebar.channelRemove", { name: nameOf(id) })}
-              className="inline-flex items-center gap-1 max-w-full pl-2.5 pr-1.5 py-1 rounded-full bg-accent text-accent-fg text-xs shadow-xs hover:opacity-90 transition"
+              className="chip-solid inline-flex items-center gap-1 max-w-full pl-2.5 pr-1.5 py-1 rounded-full text-xs shadow-xs hover:opacity-90 transition"
             >
               <span className="truncate">{nameOf(id)}</span>
               <X className="w-3 h-3 shrink-0" />