refactor(web): restructure to feature-based architecture with zustand stores

- Remove tab system entirely (tab-store, tab-bar, tab-link)
- Split monolithic AuthContext into zustand auth + workspace stores
- Move issue components/config to features/issues/
- Move WebSocket provider to features/realtime/
- Move api.ts to shared/
- Migrate all consumers from useAuth() to direct store imports
- Simplify sidebar: replace hand-built dropdown with shadcn DropdownMenu,
  replace custom layout wrapper with SidebarInset
- Remove unused @multica/store and @multica/hooks dependencies
- Add @/ path alias and zustand dependency
- Update CLAUDE.md with feature-based architecture conventions

Net change: +293 / -2435 lines

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/package.json

@@ -15,9 +15,7 @@
     "@dnd-kit/core": "^6.3.1",
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
-    "@multica/hooks": "workspace:*",
     "@multica/sdk": "workspace:*",
-    "@multica/store": "workspace:*",
     "@multica/types": "workspace:*",
     "@multica/ui": "workspace:*",
     "@multica/utils": "workspace:*",
@@ -25,7 +23,8 @@
     "next": "^16.1.6",
     "next-themes": "^0.4.6",
     "react": "catalog:",
-    "react-dom": "catalog:"
+    "react-dom": "catalog:",
+    "zustand": "catalog:"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "catalog:",