fix(ui): theme token consistency, seek bar a11y, colour polish

- Add --color-success variable to all three themes
- Replace hard-coded amber/green Tailwind colours with CSS variables:
    Pro badge + discount badge: bg-amber-400/15 → bg-(--color-brand)/15
    Saved confirmation:         text-green-400 → text-(--color-success)
    Catalogue flash messages:   emerald/yellow → success/brand tokens
    Login hover border:         border-zinc-600 → border-(--color-brand)/50
- Seek bar in mini-player now has aria-label (player_seek_label)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/app.css

@@ -9,7 +9,8 @@
 	--color-muted: #a1a1aa;       /* zinc-400 */
 	--color-text: #f4f4f5;        /* zinc-100 */
 	--color-border: #3f3f46;      /* zinc-700 */
-	--color-danger: #f87171;      /* red-400 */
+	--color-danger:  #f87171;      /* red-400 */
+	--color-success: #4ade80;      /* green-400 */
 }
 
 /* ── Amber theme (default) — same as @theme above, explicit for clarity ── */
@@ -23,6 +24,7 @@
 	--color-text:      #f4f4f5;
 	--color-border:    #3f3f46;
 	--color-danger:    #f87171;
+	--color-success:   #4ade80;
 }
 
 /* ── Slate theme — indigo/slate dark ─────────────────────────────────── */
@@ -36,6 +38,7 @@
 	--color-text:      #f1f5f9;   /* slate-100 */
 	--color-border:    #334155;   /* slate-700 */
 	--color-danger:    #f87171;   /* red-400 */
+	--color-success:   #4ade80;   /* green-400 */
 }
 
 /* ── Rose theme — dark pink ───────────────────────────────────────────── */
@@ -49,6 +52,7 @@
 	--color-text:      #f4f4f5;   /* zinc-100 */
 	--color-border:    #3f2d36;   /* custom rose border */
 	--color-danger:    #f87171;   /* red-400 */
+	--color-success:   #4ade80;   /* green-400 */
 }
 
 html {