[FEATURE]: Make V2 App prompt action shortcuts configurable

Feature hasn't been suggested before.

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Describe the enhancement you want to request

The V2 web/Desktop App has a Keyboard Shortcuts editor, but prompt submit, newline, and alternate delivery are still hardcoded and cannot be rebound. I want these actions exposed through the existing shortcut UI while preserving the current defaults:

• prompt.submit: Enter
• prompt.newline: Shift+Enter
• prompt.submit.alternate: Ctrl/Cmd+Enter — queue or steer opposite the configured follow-up behavior

This would allow configurations such as Enter for newline and Ctrl/Cmd+Enter for submit, with a separate customizable alternate-delivery shortcut, without adding another settings model. Each action should support disabling and restoring its default, and shortcut hints should reflect the effective bindings.

This request is specifically for the V2 App architecture. Related historical work does not cover the current implementation:

• #9836 concerns TUI terminal handling.
• #11898 was closed as a Question issue with instructions to open a proper feature request.
• #16226 and #23430 targeted the V1 App architecture.
• #13637 targets the V1 App, uses a boolean setting, and does not support the V2 composer.

The proposed V2 implementation registers all three actions in the App command catalog but keeps dispatch editor-local so suggestions, IME composition, selection replacement, and key-repeat behavior remain correct in the current Composer used by both new-session and session surfaces.

Start by tracing the existing Keyboard Shortcuts editor, the App command catalog, and the current Composer used by new-session and session surfaces. Verify how prompt actions are dispatched and how shortcut hints are derived, then confirm that the three actions retain their defaults, support disabling and reset, and preserve editor-local behavior for suggestions, IME composition, selection replacement, and key repeat.