Fix timeline playhead alignment and scroll-on-select

[cbaucom]

Summary

• Align included timeline thumbnails to RenderPlan start frames (same time axis as the red playhead and audio lane) instead of cumulative packed layout — fixes drift after duration changes.
• Auto-scroll follows the playhead only during playback; selecting a slide while paused scrolls that block into view once without snapping back to the stale playhead.
• Synchronously update currentFrame on thumbnail seek so the playhead moves immediately.

Closes #57

Test plan

• pnpm test / pnpm lint / pnpm build
• Change global image duration — red line stays within the green "now playing" thumbnail
• Change per-slide duration override — playhead stays aligned
• Pause, scroll to a distant thumbnail, click it — viewport stays on (or scrolls to) that slide, not back to old playhead
• Play slideshow — timeline auto-follows playhead
• Zoom timeline while a slide is selected — viewport does not jump

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