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/components/ThemeApplier.tsx

@@ -0,0 +1,16 @@
+'use client'
+
+import { useEffect } from 'react'
+import { useThemeStore, ACCENT_PRESETS } from '@/store/themeStore'
+
+export default function ThemeApplier() {
+  const { accentIdx } = useThemeStore()
+
+  useEffect(() => {
+    const preset = ACCENT_PRESETS[accentIdx] ?? ACCENT_PRESETS[0]
+    document.documentElement.style.setProperty('--accent', preset.accent)
+    document.documentElement.style.setProperty('--accent-rgb', preset.rgb)
+  }, [accentIdx])
+
+  return null
+}