feat: Add loot builder frontend — Pinia store, views, 4 components, router + nav

Implements the complete frontend for BetterLoot profile management:
- Pinia store (loot.ts) with CRUD, import/export, apply-to-server actions
- LootBuilderView orchestrator with profile bar, modals, two-column layout
- LootContainerSidebar with categorized container list, search, config indicators
- LootItemEditor for per-container item settings and ungrouped item table
- LootItemPicker modal with searchable/filterable Rust item grid
- LootGroupEditor for reusable loot group management
- Router integration at /loot-builder
- Sidebar nav item with Crosshair icon and loot.view permission

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

frontend/src/components/layout/DashboardLayout.vue

@@ -27,6 +27,7 @@ import {
   AlertTriangle,
   FileText,
   FolderOpen,
+  Crosshair,
   Menu,
   X,
 } from 'lucide-vue-next'
@@ -44,6 +45,7 @@ const navItems = [
   { name: 'Players', path: '/players', icon: Users, permission: 'players.view' },
   { name: 'Plugins', path: '/plugins', icon: Puzzle, permission: 'plugins.view' },
   { name: 'File Manager', path: '/files', icon: FolderOpen, permission: 'files.view' },
+  { name: 'Loot Builder', path: '/loot-builder', icon: Crosshair, permission: 'loot.view' },
   { name: 'Auto-Wiper', path: '/wipes', icon: RefreshCw, permission: 'wipes.view' },
   { name: 'Maps', path: '/maps', icon: Map, permission: 'maps.view' },
   { name: 'Chat Log', path: '/chat', icon: MessageSquare, permission: 'chat.view' },