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/style.css

@@ -235,12 +235,64 @@ button:disabled {
   background: none;
 }
 
-.empty-message {
+.empty-message,
+.loading-message {
   text-align: center;
   color: #999;
   padding: 40px 20px;
 }
 
+/* Asset Grid */
+.asset-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(70px, 1fr));
+  gap: 10px;
+  padding: 10px;
+  background: #f9f9f9;
+  border: 1px solid #ddd;
+  border-radius: 4px;
+}
+
+.asset-item {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  padding: 8px;
+  border: 2px solid transparent;
+  border-radius: 4px;
+  cursor: pointer;
+  transition: all 0.15s ease;
+}
+
+.asset-item:hover {
+  background: #eee;
+}
+
+.asset-item img {
+  width: 40px;
+  height: 40px;
+  object-fit: contain;
+}
+
+.asset-item .asset-name {
+  font-size: 11px;
+  color: #666;
+  margin-top: 4px;
+  text-align: center;
+  word-break: break-all;
+}
+
+.asset-item:has(input:checked) {
+  border-color: #3498db;
+  background: #e7f3fd;
+}
+
+.asset-item input[type="checkbox"] {
+  position: absolute;
+  opacity: 0;
+  pointer-events: none;
+}
+
 @media (max-width: 900px) {
   .container {
     flex-direction: column;