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/src/lib.rs

@@ -9,6 +9,7 @@ pub fn run() {
     tauri::Builder::default()
         .plugin(tauri_plugin_opener::init())
         .plugin(tauri_plugin_dialog::init())
+        .plugin(tauri_plugin_http::init())
         .invoke_handler(tauri::generate_handler![greet])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");