Desktop: dragging files into chat pastes the filesystem path instead of attaching

## Problem

On Linux (WebKitGTK) — and likely other Tauri webviews — dragging a photo or file from the file manager into the message composer inserts the **filesystem path as chat text** (`/home/…/photo.png`) instead of attaching/uploading the file.

Paperclip picker and clipboard paste of images still work. Drop is the broken path.

## Why

`tauri.conf.json` has `dragDropEnabled: false`, so OS drops go to the webview. The composer form has HTML5 `onDrop` that only reads `dataTransfer.files`.

Linux WebKitGTK often:

1. Leaves `dataTransfer.files` empty
2. Puts `file://…` / an absolute path in `text/uri-list` or `text/plain`
3. Lets ProseMirror handle the drop first (`editorProps.handleDrop` is unset), which inserts that path as text

The window-level handler only `preventDefault`s when `types` includes `Files`, so `text/uri-list` drops are not claimed.

## Expected

Dropping a JPEG/PNG/PDF/etc. onto the composer (or forum composer) should attach it the same way as the paperclip button — upload overlay, then send with the message.

## Environment

- Buzz Desktop (self-hosted relay)
- Linux
- Drop from the file manager onto the chat input

Start by reading tauri.conf.json and the window-level drop handler, then trace the chat and forum composer onDrop paths and ProseMirror editorProps.handleDrop. Compare them with the paperclip upload flow. Done means file-manager drops attach JPEG, PNG, PDF, and similar files instead of inserting paths, including in both composers.

Written by the indexing model from the issue text.