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/vite.config.js

@@ -1,9 +1,21 @@
 // Copyright (c) 2026 Gordon Bolton. MIT License.
 import { defineConfig } from 'vite'
-import react from '@vitejs/plugin-react'
 
 export default defineConfig({
-  plugins: [react()],
+  build: {
+    outDir: '../public',
+    emptyOutDir: false,
+    rollupOptions: {
+      input: {
+        main: './src/main.js'
+      },
+      output: {
+        entryFileNames: 'app.js',
+        chunkFileNames: '[name].js',
+        assetFileNames: '[name][extname]'
+      }
+    }
+  },
   server: {
     port: 5173,
     proxy: {