Add message edit and soft-delete support in message view

[Juergen-Coding]

This PR adds forum-style editing and soft-delete support in message view.

Summary:

• Add Edit and Delete options to the message footer menu
• Add key bindings:
  • E = edit current message
  • X = soft-delete current message with confirmation
• Implement edit workflow that loads an existing message and prefills the editor
• Add edit post menu flow
• Keep the changes limited to the relevant core files and message base menu template

Files changed:

• core/message.js
• core/msg_area_post_fse.js
• core/msg_area_view_fse.js
• misc/menu_templates/message_base.in.hjson

Notes:

• Soft-delete is currently implemented via subject == "[DELETED]"
• Intended for local boards
• No DB schema changes required
• Related discussion: I added edit and delete options to the message #623