fix(desktop): attach OS file drops instead of inserting their paths - #6934

Fixes #6933

Problem

Dragging a photo or file from the OS file manager into the composer inserts the filesystem path as chat text instead of attaching the file. Reproduced on Linux WebKitGTK against a self-hosted relay.

Paperclip + clipboard image paste already work. Drop does not.

Cause

• dragDropEnabled is false, so drops go to the webview.
• The form onDrop only reads dataTransfer.files.
• Linux WebKitGTK often omits File objects and instead delivers text/uri-list / text/plain with file://… or an absolute path.
• Tiptap/ProseMirror has no editorProps.handleDrop, so it inserts that path as text.

Fix

• Claim OS file drops in editorProps.handleDrop (message + forum composers) before ProseMirror inserts text, and stopPropagation so the form handler does not double-upload.
• Recover absolute paths from text/uri-list / text/plain when files is empty (require a basename with an extension so /usr/bin/env python is not treated as a file).
• Upload recovered paths through a new upload_dropped_media command that uses the same TOCTOU-safe process_picked_path pipeline as the paperclip picker (HEIC transcode, MIME deny-list, size caps).
• Treat text/uri-list as a file drag for the drop overlay and window preventDefault (avoids file:/// navigation).

Tests

cd desktop && pnpm exec node --import ./test-loader.mjs --experimental-strip-types --test \
  src/features/messages/lib/droppedFiles.test.mjs \
  src/features/messages/lib/useMediaUpload.test.mjs

40 passed (15 new path/URI cases + existing slot tests).

Rust validate_dropped_path tests added; not compiled here (GTK syslibs missing on this machine).

Manual test

1. just desktop-dev or a local Tauri build.
2. Drag a JPEG/PNG from the file manager onto the composer (onto the text area, not only the paperclip).
3. Expect the upload preview, not /home/…/photo.png in the input.
4. Send; the image should appear in the channel.
5. Drag ordinary selected text into the composer — it should still insert as text.
6. Forum composer: same drop behavior.

No UI screenshots: behavior is drop → attach; the overlay already existed.

None found for this specific path-as-text bug; closest existing work is the form-level handleDrop that only sees dataTransfer.files.