[FEATURE] Desktop app: support custom keybinds for input newline (Ctrl+Enter)

Is your feature request related to a problem? Please describe.

In the desktop app, pressing Enter immediately sends the message. There is no way to type multi-line input before sending. The terminal TUI supports this via tui.json keybind configuration (input_newline, input_submit), but the desktop app does not read tui.json.

Describe the solution you'd like

Add keybind configuration support to the desktop app, specifically:

• Enter — send message
• Ctrl+Enter / Shift+Enter — newline (queue text without sending)

This should be configurable through the same tui.json or opencode.json config that the terminal TUI uses.

Describe alternatives you've considered

• Using the terminal TUI instead (works, but less convenient)
• Modifying app.asar (breaks on updates)

Additional context

The desktop app is an Electron app that appears to use a separate configuration system from the terminal TUI. The tui.json keybinds (input_submit, input_newline) should be shared across both interfaces.