Add image/attachment support in preview mode

- Fetch attachments via WebDAV using Tauri HTTP plugin (bypasses CORS)
- Parse markdown for image references and convert to base64 data URLs
- In-memory cache to avoid re-fetching images
- Loading indicator while images load
- Register tauri-plugin-http in Rust builder
- Add HTTP permissions in capabilities

package.json

@@ -12,6 +12,7 @@
   "dependencies": {
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-dialog": "^2.6.0",
+    "@tauri-apps/plugin-http": "^2.5.7",
     "@tauri-apps/plugin-opener": "^2",
     "@tiptap/extension-strike": "^2.27.2",
     "@tiptap/extension-underline": "^2.27.2",