FR-1: Background Polling

• The client must poll for new messages on a configurable interval (default: 10 seconds)
• Polling only occurs when the user is in the messages view (not chats list, loading, or error screens)
• Polling pauses when the OSK is open
• Polling pauses when the system menu overlay is open
• Polling must not block the main input/render loop (use non-blocking timer accumulation)