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

src-tauri/capabilities/default.json

@@ -5,6 +5,14 @@
   "windows": ["main"],
   "permissions": [
     "core:default",
-    "opener:default"
+    "opener:default",
+    "http:default",
+    {
+      "identifier": "http:default",
+      "allow": [
+        { "url": "https://*" },
+        { "url": "http://*" }
+      ]
+    }
   ]
 }