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 se mencionan archivos ni tests. Empieza localizando el compositor de chat y el punto de entrada para la persistencia de la configuración o las preferencias; después, sigue cómo se gestiona actualmente el atajo de envío. Se considera terminado cuando los usuarios pueden elegir cualquiera de los dos atajos, la elección persiste entre sesiones y el atajo activo se muestra claramente.

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