feat: add frontend UI for SimpleNote Web

- Vanilla JS frontend with dark theme
- Dashboard with sidebar (libraries tree, tags), document grid, search
- Document viewer with markdown rendering and metadata panel
- Document editor with split write/preview and formatting toolbar
- Login screen with token authentication
- All styled according to UI/UX specs (dark theme, accent #00d4aa)
- API client for all endpoints
- Responsive design

public/js/api.js

@@ -0,0 +1,120 @@
+// API Client for SimpleNote Web
+
+const API_BASE = '/api/v1';
+
+class ApiClient {
+  constructor() {
+    this.token = localStorage.getItem('sn_token');
+  }
+
+  setToken(token) {
+    this.token = token;
+    if (token) {
+      localStorage.setItem('sn_token', token);
+    } else {
+      localStorage.removeItem('sn_token');
+    }
+  }
+
+  getHeaders() {
+    const headers = {
+      'Content-Type': 'application/json'
+    };
+    if (this.token) {
+      headers['Authorization'] = `Bearer ${this.token}`;
+    }
+    return headers;
+  }
+
+  async request(method, path, body = null) {
+    const options = {
+      method,
+      headers: this.getHeaders()
+    };
+    if (body) {
+      options.body = JSON.stringify(body);
+    }
+
+    const response = await fetch(`${API_BASE}${path}`, options);
+    
+    if (!response.ok) {
+      const error = await response.json().catch(() => ({ message: 'Request failed' }));
+      throw new Error(error.message || `HTTP ${response.status}`);
+    }
+
+    return response.json();
+  }
+
+  get(path) { return this.request('GET', path); }
+  post(path, body) { return this.request('POST', path, body); }
+  put(path, body) { return this.request('PUT', path, body); }
+  delete(path) { return this.request('DELETE', path); }
+
+  // Auth
+  async login(token) {
+    try {
+      const data = await this.get('/auth/verify');
+      this.setToken(token);
+      return data;
+    } catch (e) {
+      this.setToken(null);
+      throw e;
+    }
+  }
+
+  // Documents
+  getDocuments(params = {}) {
+    const query = new URLSearchParams(params).toString();
+    return this.get(`/documents${query ? '?' + query : ''}`);
+  }
+
+  getDocument(id) {
+    return this.get(`/documents/${id}`);
+  }
+
+  createDocument(data) {
+    return this.post('/documents', data);
+  }
+
+  updateDocument(id, data) {
+    return this.put(`/documents/${id}`, data);
+  }
+
+  deleteDocument(id) {
+    return this.delete(`/documents/${id}`);
+  }
+
+  exportDocument(id) {
+    return fetch(`${API_BASE}/documents/${id}/export`, {
+      headers: this.getHeaders()
+    }).then(r => r.text());
+  }
+
+  // Libraries
+  getLibraries() {
+    return this.get('/libraries');
+  }
+
+  getLibrary(id) {
+    return this.get(`/libraries/${id}`);
+  }
+
+  createLibrary(data) {
+    return this.post('/libraries', data);
+  }
+
+  updateLibrary(id, data) {
+    return this.put(`/libraries/${id}`, data);
+  }
+
+  deleteLibrary(id) {
+    return this.delete(`/libraries/${id}`);
+  }
+
+  // Tags
+  getTags() {
+    return this.get('/tags');
+  }
+}
+
+export const api = new ApiClient();