Fill timeline buffer within clip action

[KyleTryon]

Summary

• render preview ready/buffer state as an inner clip action fill instead of a free-floating overlay
• clamp ready fill percentages to the selection action bounds so the indicator cannot overhang the track
• add coverage for timeline fill percentage mapping and clamping

Tests

• pnpm --filter @cliparr/frontend test
• pnpm --filter @cliparr/frontend lint
• pnpm --filter @cliparr/frontend build