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/sse.js

@@ -0,0 +1,60 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+
+import { state, SSE_RECONNECT_MS } from './state.js';
+import { renderDownloads } from './ui/downloads.js';
+import { hideError, hideLoading } from './ui/auth.js';
+
+export function startSSE() {
+  stopSSE();
+  const params = state.showAll ? '?showAll=true' : '';
+  const source = new EventSource('/api/dashboard/stream' + params);
+  state.sseSource = source;
+
+  let firstMessage = true;
+  source.onmessage = (event) => {
+    try {
+      const data = JSON.parse(event.data);
+      state.currentUser = data.user;
+      state.isAdmin = !!data.isAdmin;
+      state.downloads = data.downloads;
+      // Store download clients and update filter dropdown
+      if (data.downloadClients) {
+        state.downloadClients = data.downloadClients;
+        // Trigger filter update
+        const filterUpdateEvent = new CustomEvent('downloadClientsUpdated');
+        document.dispatchEvent(filterUpdateEvent);
+      }
+      document.getElementById('currentUser').textContent = state.currentUser || '-';
+      renderDownloads();
+      hideError();
+      if (firstMessage) { firstMessage = false; hideLoading(); }
+    } catch (err) {
+      console.error('[SSE] Failed to parse message:', err);
+    }
+  };
+
+  source.onerror = () => {
+    // EventSource retries automatically; we just log and show a reconnecting indicator
+    console.warn('[SSE] Connection lost, browser will retry...');
+  };
+
+  console.log('[SSE] Stream connected');
+}
+
+export function stopSSE() {
+  if (state.sseReconnectTimer) { clearTimeout(state.sseReconnectTimer); state.sseReconnectTimer = null; }
+  if (state.sseSource) {
+    state.sseSource.close();
+    state.sseSource = null;
+    console.log('[SSE] Stream closed');
+  }
+}
+
+export function handleShowAllToggle(checked) {
+  state.showAll = checked;
+  // Re-open stream with updated showAll param
+  startSSE();
+  // Trigger history reload with updated showAll param
+  const historyReloadEvent = new CustomEvent('historyReload');
+  document.dispatchEvent(historyReloadEvent);
+}