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/state.js

@@ -0,0 +1,38 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+
+// Global state (using objects for mutability across modules)
+export const state = {
+  currentUser: null,
+  downloads: [],
+  downloadClients: [], // List of download clients from server (for ordering/filtering)
+  selectedDownloadClients: [], // Array of selected client IDs for multi-select filter
+  isAdmin: false,
+  showAll: false,
+  csrfToken: null, // double-submit CSRF token, sent as X-CSRF-Token on mutating requests
+  
+  // History section state
+  historyDays: 7, // Default value, will be loaded from localStorage
+  historyRefreshHandle: null,
+  ignoreAvailable: false, // Default value, will be loaded from localStorage
+  lastHistoryItems: [], // raw items from last fetch, for re-filtering without a network round-trip
+  
+  // SSE stream state
+  sseSource: null,
+  sseReconnectTimer: null,
+  
+  // Status panel state
+  statusRefreshHandle: null,
+  
+  // Webhooks state
+  webhookSectionExpanded: false,
+  webhookLoading: false,
+  sonarrWebhook: { enabled: false, triggers: { onGrab: false, onDownload: false, onImport: false, onUpgrade: false }, stats: null },
+  radarrWebhook: { enabled: false, triggers: { onGrab: false, onDownload: false, onImport: false, onUpgrade: false }, stats: null },
+  webhookMetrics: null
+};
+
+// Constants
+export const SPLASH_MIN_MS = 1200; // minimum splash display time
+export const HISTORY_REFRESH_MS = 5 * 60 * 1000; // auto-refresh history every 5 min
+export const SSE_RECONNECT_MS = 3000; // browser already retries, but we add explicit backoff too
+export const STATUS_REFRESH_MS = 5000;