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/shared/api.ts

@@ -0,0 +1,17 @@
+import { ApiClient } from "@multica/sdk";
+
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL ?? "http://localhost:8080";
+
+export const api = new ApiClient(API_BASE_URL);
+
+// Initialize token from localStorage on load
+if (typeof window !== "undefined") {
+  const token = localStorage.getItem("multica_token");
+  if (token) {
+    api.setToken(token);
+  }
+  const wsId = localStorage.getItem("multica_workspace_id");
+  if (wsId) {
+    api.setWorkspaceId(wsId);
+  }
+}