fix(dashboard): refresh multimodal samples on new steps

[xsuler]

Summary

• automatically follow the newest rollout sample when polling observes a new training step
• reset the selected sample when the job or latest step changes, while preserving manual selection between polls that do not advance the step
• remount multimodal media when the active sample changes so stale video/audio/image elements are not reused
• autoplay rollout videos inline and muted, with looping and controls for manual unmute/playback
• rebuild the packaged dashboard assets

Root cause

SampleView only changed selectedStep when the existing selection disappeared. When a new step arrived, the old step remained a valid option, so the dashboard continued rendering the previous sample. Media elements could also be reconciled across sample changes because their component identity did not include the active sample key.

Validation

• pnpm build: passed
• git diff --check: passed
• targeted dashboard Python tests could not be collected in the local environment because PyTorch is not installed (ModuleNotFoundError: torch)