feat: runtime accent color picker with 6 presets

Replace all hardcoded #c8ff00 / rgba(200,255,0,...) with CSS custom
properties (--accent, --accent-rgb) so the accent color updates live.
Add themeStore (Zustand + localStorage) with 6 presets (Лайм, Синий,
Розовый, Фиолет, Оранж, Минт). Add ThemeApplier component that syncs
CSS vars on load. Add color picker UI section in ExtraTab.

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

apps/web/src/app/page.tsx

@@ -120,7 +120,7 @@ export default function LandingPage() {
         <div className="flex items-center gap-3 flex-wrap justify-center">
           <Link
             href="/app"
-            className="px-8 py-3 bg-accent text-bg font-display font-bold text-sm rounded-xl hover:brightness-110 active:scale-[0.97] transition-all duration-150 shadow-[0_0_24px_rgba(200,255,0,0.25)]"
+            className="px-8 py-3 bg-accent text-bg font-display font-bold text-sm rounded-xl hover:brightness-110 active:scale-[0.97] transition-all duration-150 shadow-[0_0_24px_rgba(var(--accent-rgb),0.25)]"
           >
             Начать вечеринку →
           </Link>