Migrate frontend from monolithic app.js to vanilla ES modules

- Delete React files (App.jsx, main.jsx, App.css)
- Create modular vanilla JS structure in client/src/:
  - state.js (global state object)
  - api.js (all fetch calls)
  - sse.js (SSE connection management)
  - ui/auth.js (authentication UI)
  - ui/downloads.js (downloads rendering)
  - ui/history.js (history section)
  - ui/statusPanel.js (status panel)
  - ui/webhooks.js (webhook management)
  - ui/filters.js (download client filter)
  - ui/theme.js (theme switching)
  - ui/tabs.js (tab navigation)
  - utils/format.js (formatting utilities)
  - utils/storage.js (localStorage helpers)
  - main.js (DOMContentLoaded bootstrap)
- Update vite.config.js for vanilla build outputting to ../public/app.js
- Build succeeds: 14 modules, 43.88 kB output

client/src/main.js

@@ -0,0 +1,62 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+
+// Bootstrap - wire all event handlers on DOMContentLoaded
+import { checkAuthenticationAndInit, handleLogin, handleLogoutClick } from './ui/auth.js';
+import { initDownloadClientFilter } from './ui/filters.js';
+import { initHistoryControls } from './ui/history.js';
+import { toggleStatusPanel } from './ui/statusPanel.js';
+import { initWebhooks } from './ui/webhooks.js';
+import { initThemeSwitcher } from './ui/theme.js';
+import { initTabs, goHome } from './ui/tabs.js';
+import { handleShowAllToggle } from './sse.js';
+import { loadAppVersion } from './api.js';
+
+document.addEventListener('DOMContentLoaded', () => {
+  // Login form
+  const loginForm = document.getElementById('login-form');
+  if (loginForm) {
+    loginForm.addEventListener('submit', handleLogin);
+  }
+  
+  // Logout button
+  const logoutBtn = document.getElementById('logout-btn');
+  if (logoutBtn) {
+    logoutBtn.addEventListener('click', handleLogoutClick);
+  }
+  
+  // Show all toggle
+  const showAllToggle = document.getElementById('show-all-toggle');
+  if (showAllToggle) {
+    showAllToggle.addEventListener('change', (e) => handleShowAllToggle(e.target.checked));
+  }
+  
+  // Status panel toggle
+  const statusToggle = document.getElementById('status-toggle');
+  if (statusToggle) {
+    statusToggle.addEventListener('click', toggleStatusPanel);
+  }
+  
+  // Home button
+  const homeBtn = document.getElementById('home-btn');
+  if (homeBtn) {
+    homeBtn.addEventListener('click', goHome);
+  }
+  
+  // Initialize UI components
+  initThemeSwitcher();
+  initTabs();
+  initDownloadClientFilter();
+  initHistoryControls();
+  initWebhooks();
+  
+  // Load app version
+  loadAppVersion().then(version => {
+    const versionEl = document.getElementById('app-version');
+    if (versionEl && version) {
+      versionEl.textContent = 'v' + version;
+    }
+  });
+  
+  // Check authentication and initialize
+  checkAuthenticationAndInit();
+});