feat(theme): R10 S1 — semantic status colors (danger/warn/success)

Add theme-scoped --danger/--warn/--success (+ -fg) tokens wired into the v4
@theme inline, replacing ~110 raw red/amber/emerald/green palette refs across
34 files. Fixes light-theme contrast failures (fixed Tailwind shades fell to
~1.7-3:1 on white). Status colors are scheme-independent — recognised by hue,
shifting only by theme to hold contrast (bright on dark bg, darkened on light).
Solid fills use the paired -fg text; @source inline guarantees the full -fg set
exists. IMDb rating gold kept as a deliberate decorative non-status exception.

frontend/src/components/PlaylistsRail.tsx

@@ -216,7 +216,7 @@ export default function PlaylistsRail({
               <button
                 type="button"
                 onClick={() => listQuery.refetch()}
-                className="text-xs text-red-400 px-1 py-1 text-left hover:underline"
+                className="text-xs text-danger px-1 py-1 text-left hover:underline"
               >
                 {t("common.loadError")} — {t("common.retry")}
               </button>