FR-5: Optimistic Send

• When the user sends a message, immediately append it to self.messages with a temporary GUID
• Render it in the outgoing bubble style with a subtle visual distinction (e.g., a small clock icon or lighter blue) to indicate “pending”
• When the poll/next fetch returns the confirmed message (with real GUID from BlueBubbles), replace the optimistic entry
• If send fails, mark the optimistic message with an error indicator (e.g., red border or retry icon)