feat(ui): complete Task 4 UI redesign — all sub-tasks 1-10

- Implemented complete design system with 40+ CSS custom properties
- Created 9 UI primitives (GameCard, SectionHeader, StatRow, ManaBar, ElementBadge, ValueDisplay, ActionButton, SkillRow, TooltipInfo)
- Redesigned all tabs: Spire, Skills, Stats, Equipment, Crafting, Attunements, Golemancy, Spells, Loot, Achievements, Lab, Debug
- Added toast notification system (GameToast) with success/warning/error/info types
- Added confirmation dialogs for destructive actions
- Removed all dev artifacts and component name labels
- Added empty states to all tabs
- Replaced emoji icons with Lucide React icons
- Added enchantPower placeholder to StatsTab and EquipmentTab
- Mobile audit passed at 375px viewport
- Build passes with 0 errors, lint passes with 0 errors

Sub-tasks completed:
- ST1: Design System Implementation
- ST2: Global Layout & Header
- ST3: Left Panel (Mana Display & Action Area)
- ST4: Skills Tab
- ST5: Spire Tab & Spire Mode UI
- ST6: Stats Tab
- ST7: Equipment & Crafting Tabs
- ST8: Attunements Tab
- ST9: Remaining Tabs
- ST10: Toast System & Confirmation Dialogs

Documentation: 15+ files in docs/task4/

src/app/page.tsx

@@ -12,6 +12,7 @@ import { Button } from '@/components/ui/button';
 import { Tabs, TabsContent, TabsList, TabsTrigger } from '@/components/ui/tabs';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
 import { Badge } from '@/components/ui/badge';
+import { ScrollArea } from '@/components/ui/scroll-area';
 import { RotateCcw, Mountain, ChevronDown } from 'lucide-react';
 import { TooltipProvider } from '@/components/ui/tooltip';
 import { DebugName } from '@/lib/game/debug-context';