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/ui/tabs.js

@@ -0,0 +1,50 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+
+import { getActiveTab, saveActiveTab } from '../utils/storage.js';
+import { loadHistory } from './history.js';
+
+export function initTabs() {
+  const downloadsTab = document.getElementById('downloads-tab');
+  const historyTab = document.getElementById('history-tab');
+  const downloadsSection = document.getElementById('downloads-section');
+  const historySection = document.getElementById('history-section');
+  
+  if (!downloadsTab || !historyTab) return;
+  
+  // Load saved tab
+  const savedTab = getActiveTab();
+  if (savedTab === 'history') {
+    activateTab('history');
+  } else {
+    activateTab('downloads');
+  }
+  
+  downloadsTab.addEventListener('click', () => activateTab('downloads'));
+  historyTab.addEventListener('click', () => activateTab('history'));
+}
+
+export function activateTab(tab) {
+  const downloadsTab = document.getElementById('downloads-tab');
+  const historyTab = document.getElementById('history-tab');
+  const downloadsSection = document.getElementById('downloads-section');
+  const historySection = document.getElementById('history-section');
+  
+  if (tab === 'downloads') {
+    downloadsTab.classList.add('active');
+    historyTab.classList.remove('active');
+    downloadsSection.style.display = 'block';
+    historySection.style.display = 'none';
+    saveActiveTab('downloads');
+  } else if (tab === 'history') {
+    historyTab.classList.add('active');
+    downloadsTab.classList.remove('active');
+    historySection.style.display = 'block';
+    downloadsSection.style.display = 'none';
+    saveActiveTab('history');
+    loadHistory();
+  }
+}
+
+export function goHome() {
+  activateTab('downloads');
+}