Add configurable multiline input behavior to chat

### Feature summary

Please add an option for easier multiline prompt editing in chat.

### What problem are you trying to solve?

Writing multiline prompts is difficult because pressing `Enter` immediately sends the message instead of adding a new line.

**Proposed behavior:**

- `Enter` inserts a new line.

- `Ctrl+Enter` sends the prompt.

- Add a general setting or toggle to the chat so users can choose between this behavior and the current `Enter to send` behavior.

**Why:**

This would make writing and editing longer, structured prompts easier while accommodating different user preferences.

**Acceptance criteria:**

- Users can select their preferred send shortcut in Settings or with the toggle.

- The selected behavior persists across sessions.

- The UI clearly indicates the active send shortcut.

### Proposed solution

- `Enter` inserts a new line.

- `Ctrl+Enter` sends the prompt.

- Add a general setting or toggle to the chat so users can choose between this behavior and the current `Enter to send` behavior.

### Workflow impact

This would make longer, structured prompts easier to write and reduce accidental submissions, while preserving each user’s preferred workflow.

### Installation context

_No response_

### Additional context

_No response_

No files or tests are named. Start by locating the chat composer and the settings or preference persistence entry point, then trace how the current send shortcut is handled. Done means users can choose either shortcut, the choice persists across sessions, and the active shortcut is clearly shown.

Written by the indexing model from the issue text.