Fix base64-encoded HTML email bodies not rendering; v1.1.18

Inbound emails from some clients (Outlook, mobile) send htmlBody as
base64. Decode it on the server before storing (future emails) and on
the client before passing to the iframe srcDoc (existing stored emails).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "1.1.17",
+  "version": "1.1.18",
   "type": "module",
   "scripts": {
     "dev": "vite",