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/utils/format.js

@@ -0,0 +1,74 @@
+// Copyright (c) 2026 Gordon Bolton. MIT License.
+
+export function formatSize(size) {
+  if (!size) return 'N/A';
+  // If already a formatted string (e.g., "21.5 GB"), return as-is
+  if (typeof size === 'string') {
+    return size;
+  }
+  // If it's a number (bytes), format it
+  const sizes = ['B', 'KB', 'MB', 'GB', 'TB'];
+  const i = Math.floor(Math.log(size) / Math.log(1024));
+  return Math.round(size / Math.pow(1024, i) * 100) / 100 + ' ' + sizes[i];
+}
+
+export function formatSpeed(bytesPerSecond) {
+  if (!bytesPerSecond || bytesPerSecond === 0) return '0 B/s';
+  
+  const units = ['B/s', 'KB/s', 'MB/s', 'GB/s'];
+  let value = bytesPerSecond;
+  let unitIndex = 0;
+  
+  while (value >= 1024 && unitIndex < units.length - 1) {
+    value /= 1024;
+    unitIndex++;
+  }
+  
+  return `${value.toFixed(2)} ${units[unitIndex]}`;
+}
+
+export function formatDate(dateString) {
+  if (!dateString) return 'N/A';
+  return new Date(dateString).toLocaleString();
+}
+
+export function formatTimeAgo(timestamp) {
+  if (!timestamp) return 'Never';
+  const seconds = Math.floor((Date.now() - timestamp) / 1000);
+  if (seconds < 60) return seconds + 's ago';
+  const minutes = Math.floor(seconds / 60);
+  if (minutes < 60) return minutes + 'm ago';
+  const hours = Math.floor(minutes / 60);
+  if (hours < 24) return hours + 'h ago';
+  return Math.floor(hours / 24) + 'd ago';
+}
+
+export function escapeHtml(str) {
+  const div = document.createElement('div');
+  div.textContent = str;
+  return div.innerHTML;
+}
+
+// Build an episode-info element for series downloads/history.
+// Single episode: "S01E05 — Episode Title"
+// Multiple episodes: "Multiple episodes" with tooltip listing them all.
+// Returns null if no episode data.
+export function formatEpisodeInfo(episodes) {
+  if (!episodes || episodes.length === 0) return null;
+  const el = document.createElement('p');
+  el.className = 'episode-info';
+  if (episodes.length === 1) {
+    const ep = episodes[0];
+    const code = 'S' + String(ep.season).padStart(2, '0') + 'E' + String(ep.episode).padStart(2, '0');
+    el.textContent = ep.title ? code + ' \u2014 ' + ep.title : code;
+  } else {
+    el.textContent = 'Multiple episodes';
+    el.classList.add('multi-episode');
+    const lines = episodes.map(ep => {
+      const code = 'S' + String(ep.season).padStart(2, '0') + 'E' + String(ep.episode).padStart(2, '0');
+      return ep.title ? code + ' \u2014 ' + ep.title : code;
+    });
+    el.setAttribute('data-tooltip', lines.join('\n'));
+  }
+  return el;
+}