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/features/auth/index.ts

@@ -0,0 +1,2 @@
+export { useAuthStore } from "./store";
+export { AuthInitializer } from "./initializer";

apps/web/features/auth/initializer.tsx

@@ -0,0 +1,32 @@
+"use client";
+
+import { useEffect, type ReactNode } from "react";
+import { useAuthStore } from "./store";
+import { useWorkspaceStore } from "@/features/workspace";
+import { api } from "@/shared/api";
+
+/**
+ * Initializes auth + workspace state from localStorage on mount.
+ * Must wrap the app to ensure stores are hydrated before children render.
+ */
+export function AuthInitializer({ children }: { children: ReactNode }) {
+  const initialize = useAuthStore((s) => s.initialize);
+  const user = useAuthStore((s) => s.user);
+  const isLoading = useAuthStore((s) => s.isLoading);
+  const hydrateWorkspace = useWorkspaceStore((s) => s.hydrateWorkspace);
+
+  useEffect(() => {
+    initialize();
+  }, [initialize]);
+
+  useEffect(() => {
+    if (isLoading || !user) return;
+    const wsId = localStorage.getItem("multica_workspace_id");
+
+    api.listWorkspaces().then((wsList) => {
+      hydrateWorkspace(wsList, wsId);
+    }).catch(console.error);
+  }, [user, isLoading, hydrateWorkspace]);
+
+  return <>{children}</>;
+}

apps/web/features/auth/store.ts

@@ -0,0 +1,61 @@
+"use client";
+
+import { create } from "zustand";
+import type { User } from "@multica/types";
+import { api } from "@/shared/api";
+
+interface AuthState {
+  user: User | null;
+  isLoading: boolean;
+
+  initialize: () => Promise<void>;
+  login: (email: string, name?: string) => Promise<User>;
+  logout: () => void;
+  setUser: (user: User) => void;
+}
+
+export const useAuthStore = create<AuthState>((set) => ({
+  user: null,
+  isLoading: true,
+
+  initialize: async () => {
+    const token = localStorage.getItem("multica_token");
+    if (!token) {
+      set({ isLoading: false });
+      return;
+    }
+
+    api.setToken(token);
+
+    try {
+      const user = await api.getMe();
+      set({ user, isLoading: false });
+    } catch {
+      api.setToken(null);
+      api.setWorkspaceId(null);
+      localStorage.removeItem("multica_token");
+      localStorage.removeItem("multica_workspace_id");
+      set({ user: null, isLoading: false });
+    }
+  },
+
+  login: async (email: string, name?: string) => {
+    const { token, user } = await api.login(email, name);
+    localStorage.setItem("multica_token", token);
+    api.setToken(token);
+    set({ user });
+    return user;
+  },
+
+  logout: () => {
+    localStorage.removeItem("multica_token");
+    localStorage.removeItem("multica_workspace_id");
+    api.setToken(null);
+    api.setWorkspaceId(null);
+    set({ user: null });
+  },
+
+  setUser: (user: User) => {
+    set({ user });
+  },
+}));