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/PlexPlaylistView.tsx

@@ -505,9 +505,8 @@ export default function PlexPlaylistView({
       title={label}
       aria-label={label}
       aria-pressed={layout === v}
-      className={`rounded-lg p-2 text-sm transition ${
-        layout === v ? "bg-accent/15 text-accent" : "glass-card glass-hover text-muted"
-      }`}
+      data-soft="true"
+      className={`pill rounded-lg p-2 text-sm ${layout === v ? "" : "glass-card glass-hover text-muted"}`}
     >
       {icon}
     </button>