feat(capture): add animation recording and unified PNG capture infras…

[patrickoleary]

Adds animation recording and single-panel PNG capture with shared infrastructure:

• Screen capture stream, cropping, and cursor/tooltip hiding shared across both paths
• Animation playback and recording share stepping logic via _step_to()
• Animation capture trigger added to Animation component in toolbars.py
• WebM video encoding via MediaRecorder included in ZIP with PNG frames

closes #23

[jourdain]

closing as it is included in #55