Add footer asset selector for email customization

- New /api/assets endpoint to list available icons from assets folder
- Dynamic footer generation with multiple selectable icons
- Thumbnail grid UI for asset selection in the mail form
- Selected icons are embedded as CID attachments in emails

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

public/script.js

@@ -4,21 +4,28 @@ document.addEventListener('DOMContentLoaded', () => {
   const submitBtn = document.getElementById('submitBtn');
   const historyList = document.getElementById('historyList');
   const clearHistoryBtn = document.getElementById('clearHistory');
+  const assetGrid = document.getElementById('assetGrid');
 
-  // Load history on page load
+  // Load history and assets on page load
   loadHistory();
+  loadAssets();
 
   // Form submission
   form.addEventListener('submit', async (e) => {
     e.preventDefault();
 
     const formData = new FormData(form);
+    const selectedAssets = Array.from(
+      document.querySelectorAll('input[name="footerAssets"]:checked')
+    ).map(cb => cb.value);
+
     const data = {
       to: formData.get('to'),
       cc: formData.get('cc') || undefined,
       subject: formData.get('subject'),
       body: formData.get('body'),
-      isHtml: formData.get('format') === 'html'
+      isHtml: formData.get('format') === 'html',
+      footerAssets: selectedAssets
     };
 
     submitBtn.disabled = true;
@@ -80,6 +87,42 @@ document.addEventListener('DOMContentLoaded', () => {
     }
   }
 
+  // Load assets from server
+  async function loadAssets() {
+    try {
+      const response = await fetch('/api/assets');
+      const result = await response.json();
+
+      if (result.success) {
+        renderAssetGrid(result.assets);
+      } else {
+        assetGrid.innerHTML = '<p class="empty-message">Fehler beim Laden der Assets</p>';
+      }
+    } catch (error) {
+      console.error('Error loading assets:', error);
+      assetGrid.innerHTML = '<p class="empty-message">Fehler beim Laden der Assets</p>';
+    }
+  }
+
+  // Render asset grid
+  function renderAssetGrid(assets) {
+    if (!assets || assets.length === 0) {
+      assetGrid.innerHTML = '<p class="empty-message">Keine Assets verfügbar</p>';
+      return;
+    }
+
+    assetGrid.innerHTML = assets.map(filename => {
+      const name = filename.replace(/\.[^.]+$/, '');
+      return `
+        <label class="asset-item">
+          <input type="checkbox" name="footerAssets" value="${filename}">
+          <img src="/assets/${filename}" alt="${name}">
+          <span class="asset-name">${name}</span>
+        </label>
+      `;
+    }).join('');
+  }
+
   // Render history list
   function renderHistory(history) {
     if (!history || history.length === 0) {