Linux: pasting an image into the composer freezes or silently does nothing

**Describe the bug**

On Linux, pasting an image from the system clipboard into the message composer does not create an attachment. With the stock AppImage the UI freezes for a while; on one attempt the bundled WebKit web process crashed. After switching the client to native Wayland and also testing the current system WebKitGTK, paste still completed with no image attached.

The same image clipboard content pastes successfully into other Linux applications and into Buzz on macOS.

**Steps to reproduce**

1. Run the Buzz desktop client on a Wayland session.
2. Copy a screenshot or PNG image to the system clipboard.
3. Focus a channel or DM message composer.
4. Press Ctrl+V.

**Expected behavior**

The image is added to the composer as an attachment/preview and the UI remains responsive.

**Actual behavior**

No image is attached. The stock AppImage pauses/freezes; the system journal repeatedly reports that the requested MIME type was not offered. One attempt also crashed the AppImage-bundled `WebKitWebProcess`.

**Version and platform**

- Buzz version: 0.5.3, `Buzz_0.5.3_amd64.AppImage`
- OS: CachyOS, kernel `7.1.5-1-cachyos`
- Desktop session: COSMIC on Wayland
- GPU: AMD Radeon RX 9070 XT (Navi 48)
- System WebKitGTK tested: `webkit2gtk-4.1 2.52.5-2`

**Logs / additional context**

While an image was on the Wayland clipboard, `wl-paste --list-types` advertised `image/png`.

The stock AppImage produces repeated COSMIC/XWayland warnings:

```
cosmic-comp: Mime type requested by X client not offered
```

One failed paste produced this coredump:

```
WebKitWebProcess SIGSEGV
/tmp/.mount_Buzz_*/usr/lib/x86_64-linux-gnu/webkit2gtk-4.1/WebKitWebProcess
```

I also tested `WEBKIT_DISABLE_DMABUF_RENDERER=1`, native Wayland instead of the AppImage's forced X11/XWayland path, and the official 0.5.4 Debian payload linked to system WebKitGTK. None made image paste work.

This is related to #2338, but is not limited to the dmabuf crash: even when there is no crash, image paste silently produces no attachment.