New component: chat message (bubble)

[fabianhueske]

New mt-chat-message component: chat bubble/message.

• Slot-driven message body (text/rich content)
• Sender roles/alignment (own/other, optional system/assistant)
• Optional author, avatar, timestamp
• Optional delivery/read status + pending/error (retry)
• Grouping of consecutive same-sender messages
• Optional attachments + per-message actions (slot)
• Sender/order/status exposed to a11y (not by alignment/color alone)