feat: add undo/fork buttons on user messages and show agent description in prompt placeholder

[cooooooooooode]

Feature request

Is your feature request related to a problem?

Currently there is no way to undo (revert) a user message or fork a conversation from a specific message in the web UI. Users have to start a new session manually. Additionally, the prompt input placeholder does not reflect which agent is currently selected.

Describe the solution you'd like

1. Add undo and fork action buttons on user messages:

   • Undo: aborts the running session and reverts the message
   • Fork: creates a new session branching from the selected message (preserving it)
   • Both actions use a two-step confirmation (click to highlight → tooltip shows confirm text → 3s timeout resets)

2. Show the current agent's description as the prompt input placeholder in the format "I am {name}, {description}" with full i18n support.

Describe alternatives you've considered

Using keyboard shortcuts or a context menu, but inline buttons are more discoverable.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Desktop/Web: Missing Revert and Fork actions for messages #9661: Desktop/Web: Missing Revert and Fork actions for messages
• Use agent description as prompt placeholder text #8804: Use agent description as prompt placeholder text
• [FEATURE]:Allow questions (or any user choice or input) to be valid fork/undo points #8589: Allow questions (or any user choice or input) to be valid fork/undo points
• [FEATURE]: ability to revert/fork from the ai message, not only from the user's #8689: Ability to revert/fork from the AI message, not only from the user's
• [FEATURE]: message actions on AI messages (copy button etc) #7696: Message actions on AI messages (copy button etc)
• [FEATURE]: Better controls/UX for queued messages. #8685: Better controls/UX for queued messages
• [FEATURE]: duplicate slash command or fork command dialog should show Duplicate at the end of message list #12001: Duplicate slash command or fork command dialog improvements
• TUI: make session fork more discoverable (and allow forking current session) #12580: TUI: make session fork more discoverable

Before proceeding, please verify these aren't already covering your feature request or related work in progress.