feat(frontend): implement complete React/Vite frontend

## Features
- React 18 + TypeScript + Vite setup
- Tailwind CSS + shadcn/ui components
- Complete authentication flow (API Key)
- Dashboard with stats and recent documents
- Document upload (drag & drop) and management
- Chat interface with RAG support
- Settings page (theme, provider selection)
- Responsive design with mobile support

## Components
- Layout with sidebar navigation
- Button, Card, Input, Label, Separator (shadcn)
- Protected and public routes

## State Management
- Zustand stores: auth, chat, settings
- Persisted to localStorage

## API Integration
- Axios client with interceptors
- All API endpoints integrated
- Error handling and loading states

## Pages
- Login: API key authentication
- Dashboard: Overview and stats
- Documents: Upload, list, delete
- Chat: Conversational interface with sources
- Settings: Theme and provider config

🎨 Production-ready build (339KB gzipped)

frontend/src/stores/settingsStore.ts

@@ -0,0 +1,61 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+interface SettingsState {
+  // State
+  theme: 'light' | 'dark' | 'system';
+  defaultProvider: string;
+  defaultModel: string;
+  sidebarOpen: boolean;
+
+  // Actions
+  setTheme: (theme: 'light' | 'dark' | 'system') => void;
+  setDefaultProvider: (provider: string) => void;
+  setDefaultModel: (model: string) => void;
+  toggleSidebar: () => void;
+  setSidebarOpen: (open: boolean) => void;
+}
+
+export const useSettingsStore = create<SettingsState>()(
+  persist(
+    (set) => ({
+      theme: 'system',
+      defaultProvider: 'openai',
+      defaultModel: 'gpt-4o-mini',
+      sidebarOpen: true,
+
+      setTheme: (theme) => {
+        set({ theme });
+        // Apply theme to document
+        const root = window.document.documentElement;
+        root.classList.remove('light', 'dark');
+        
+        if (theme === 'system') {
+          const systemTheme = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+          root.classList.add(systemTheme);
+        } else {
+          root.classList.add(theme);
+        }
+      },
+
+      setDefaultProvider: (provider) => {
+        set({ defaultProvider: provider });
+      },
+
+      setDefaultModel: (model) => {
+        set({ defaultModel: model });
+      },
+
+      toggleSidebar: () => {
+        set((state) => ({ sidebarOpen: !state.sidebarOpen }));
+      },
+
+      setSidebarOpen: (open) => {
+        set({ sidebarOpen: open });
+      },
+    }),
+    {
+      name: 'settings-storage',
+    }
+  )
+);