Make prompt submit/newline keys configurable (Enter for newline, Ctrl+Enter to submit)

[AyoubAZ-0]

Before submitting

• I searched existing issues and did not find an exact duplicate.

• I am describing a concrete problem or use case, not just a vague idea.

Area
apps/web

Problem or use case
The chat prompt input submits immediately on Enter, and there is no way to configure this behavior. Users accustomed to chat-style UX (Slack, Discord, ChatGPT web) expect Enter to insert a newline and Ctrl+Enter to submit.

This causes frequent accidental submissions when composing multi-line prompts — pasting code snippets, writing detailed task descriptions, or drafting prompts incrementally — since pressing Enter mid-edit fires the message prematurely. The only current workaround is Shift+Enter for newline, which is undiscoverable and doesn't match the muscle memory of many users.

Steps to reproduce:

1. Start typing a multi-line prompt in the chat input.
2. Press Enter to move to a new line.
3. Observe that the message is submitted immediately instead of inserting a line break.

Expected behavior
Users should be able to choose their submit/newline key mapping, e.g.:

• Option A (configurable): a Settings option or keybindings entry to swap the two actions — Enter inserts a newline, Ctrl+Enter (and/or Shift+Enter) submits.
• Option B (sensible default): Enter always inserts a newline; submission uses an explicit modifier combo such as Ctrl+Enter.

Actual behavior
Enter is hardcoded to submit. There is no setting, config file entry, or keybinding customization exposed for the prompt input's submit action.

Suggested behavior

1. Expose submit and newline as configurable keybindings in Settings, with defaults preserved (Enter = submit, Shift+Enter = newline).
2. Persist the choice per user (or workspace) alongside other editor settings.
3. Update the input placeholder/hint text so the active submit shortcut is discoverable after remapping.

Why this matters
Accidental submissions interrupt agent runs, pollute thread history, and force users to stop generation and re-paste content. This behavior is standard in comparable tools (Claude Code, Codex, Slack, Discord all offer either this exact layout or a toggle), and its absence is a daily friction point for multi-line prompt workflows.

Acceptance criteria

• Submit and newline actions are both rebindable via settings.
• With the swapped layout, Enter reliably inserts a newline in the prompt input on Windows/macOS/Linux.
• The alternative submit combo (Ctrl+Enter) works regardless of keyboard layout/IME state.
• UI hints reflect the configured binding.