feat(session): add undo functionality to session messages and context menu

[raksix]

What does this PR do?

• Adds a "Revert to here" (Undo) functionality to individual session messages.
• Introduces a Desktop-friendly Context Menu (right-click) on user messages for quick actions.
• Implements an intelligent revert logic:
  • Reverting from an older message keeps that message but removes all subsequent turns.
  • Reverting from the latest message acts as a classic "Undo" (removes the message itself).
  • Automatically restores the deleted message's content back into the prompt input for easy editing.
• Enhances the UI by replacing cluttered hover buttons with a clean, native-feeling context menu.
• Fixes a critical desktop startup issue by switching from interactive (-il) to login (-l) shell mode to prevent shell-config blocking. How did you verify your code works?
• Verified the desktop app starts correctly without hanging during the sidecar spawn process.
• Tested the context menu on various messages (both old and latest).
• Confirmed that the "Undo" correctly clears the session state and restores the prompt in the input field.
• Verified that right-clicking opens the menu at the correct location and "Copy message" works as expected.
• Ran typechecks and linting to ensure no regressions were introduced.