Harden bird screen follow-up

[SamBroner]

Summary

• fix render-cache prewarm worker lifecycle and skip uncacheable transit prewarm
• keep touch polling alive across transient MPR121/I2C errors
• let bird screens render startup loading/empty states before BirdNET data arrives
• split BirdService last-good data from notification state and bound shutdown joins
• remove dead birdstuff/hello/static-image assets and update public repo docs/tests

Tests

• uv run pytest