app: prompt submit and newline shortcuts are not configurable #42981

Summary

The V2 web/Desktop App exposes a Keyboard Shortcuts editor, but prompt submit and newline are still hardcoded and cannot be rebound.

This prevents using Enter for newline and Ctrl/Cmd+Enter for submit while preserving the existing defaults for other users.

Environment

• opencode version: v0.0.0-next-17444
• OS: macOS 27.0, arm64
• Terminal: xterm-256color
• Shell: fish
• Install/channel: next
• Active plugins: None configured

Reproduction

1. Open the V2 web or Desktop App.
2. Open Settings -> Keyboard Shortcuts.
3. Search for prompt submit or newline.
4. Observe that neither action is available.
5. Focus the prompt editor and press Enter or Shift+Enter.

Expected Behavior

Prompt submit and newline should be configurable through the existing App Keyboard Shortcuts UI, while retaining the current defaults.

Actual Behavior

Enter submission and Shift+Enter newline behavior are handled directly by both App composers and have no configurable shortcut actions.

Additional Context

At origin/v2 commit d4f10fa9bed3c19d3e28fb90f20dc208226eeee7:

• The legacy App composer handles Enter directly in packages/app/src/components/prompt-input.tsx.
• The V2 composer handles Enter directly in packages/session-ui/src/v2/components/prompt-input/index.tsx.
• V2 TUI has configurable input.submit and input.newline bindings, but its keymap and cli.json settings are separate from the App command registry and browser-local settings.

Prior context: #16226, #23430, #13637, #9836.