fix: format trim times as minutes and seconds

[gTanusri]

Summary

• Add a formatTrimTime helper for MM:SS.d trim display values
• Show formatted start/end labels and clip summary in the trim control
• Keep trim inputs numeric so users can still edit raw seconds
• Add formatter unit tests for padding, decimals, and invalid values

Tests

• Not run: local dependency install is blocked in this environment
  • npm install fails with an existing React 19 / @testing-library/react 14 peer dependency conflict
  • npm install --legacy-peer-deps fails with ENOSPC: no space left on device

Closes #78

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