Feat: Add Keyboard Shortcuts modal to Embedded Chat options menu

Feature

This feature closes that gap by adding a "Keyboard Shortcuts" option to the Embedded Chat options menu, ensuring a consistent and complete experience between the two surfaces i.e embedded chat and rocket chat.

Proposed Implementation

Add a "Keyboard Shortcuts" entry to the options menu in Embedded Chat. Clicking it opens a modal displaying the following shortcuts:

• Open Channel / User Search ⌘ / Ctrl + K or ⌘ / Ctrl + P
• Edit previous message Arrow + ↑ (when input is empty)
• Move to beginning of line ⌘ / Alt + ←
• Move to beginning of message ⌘ / Alt + ↑
• Move to end of line ⌘ / Alt + →
• Move to end of message ⌘ / Alt + ↓
• New line in message input Shift + Enter

The idea is to replicate the working of keyboard shortcuts of rocket chat in embedded chat
as how rocket chat do it

https://github.com/user-attachments/assets/a544fbb4-a2ce-4de3-99a5-911abd324eca

Start by locating the Embedded Chat options menu entry point and compare it with the existing Rocket.Chat keyboard-shortcuts experience. Verify that the listed shortcuts are displayed in a modal opened from that menu, and that the modal and all seven shortcut descriptions are present and usable.