Fix thread safety and race condition in Flask mode

[b0x42]

Summary

• Initialize image_bytes before starting the Flask server to prevent AttributeError on early requests (race condition where browser hits /screen.png before the update loop runs)
• Add threading.Lock to protect shared mutable state (image, draw, image_bytes) from concurrent access by the main thread, update loop thread, and Flask request handler thread
• Remove overly broad try/except Exception in the /screen.png route now that the root cause is fixed

Test plan

• All 99 existing tests pass
• Manual: start the emulator in Flask mode and verify the first page load renders correctly (no 500 error)
• Manual: rapidly call drawing methods while the Flask UI is open to verify no image corruption

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