desktop: drag-drop / Open With file input

[RichardBray]

Sub-issue of #133.

Scope

• Replace UploadZone's <input type="file">.click() trick with Electrobun's native Utils.openFileDialog() RPC (called in the Bun main process, chosen path(s) returned to the UI over RPC). <input type="file"> does not trigger a native picker in Electrobun's WKWebView - confirmed known upstream bug, File upload doesn't work blackboardsh/electrobun#66.
• macOS "Open With" file association support.
• Drag-drop into the window: Electrobun's native DnD is unreliable/unresolved upstream (Native drag and drop blackboardsh/electrobun#63 - inbound drop may fire but paths are reported obfuscated/unusable, outbound drag unsupported). Treat as best-effort; don't block this issue on it. If it doesn't work reliably, drop it from scope and rely on openFileDialog + Open With only.
• Watch for related upstream RPC bugs that could bite the openFileDialog integration: electrobun#156 (deadlock in async RPC handler), electrobun#449 (dialog can't close after RPC timeout), electrobun#414 (fails on paths with commas).
• Wires into the in-process FFmpeg pipeline (depends on wiring issue desktop: wire packages/ui to in-process FFmpeg calls #137).