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/Cargo.toml

@@ -23,4 +23,5 @@ tauri-plugin-opener = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tauri-plugin-dialog = "2.6.0"
+tauri-plugin-http = "2.5.7"