fix(report): stabilize record timeline scaling

[EAGzzyCSL]

Summary

• Extract timeline scale calculation into a focused helper with unit coverage.
• Use a stable px/ms scale plus nice tick steps for the Record timeline.
• Extend the visible time range to the next tick so the last screenshot thumbnail still has room to render.
• Format sub-second timeline labels in milliseconds to avoid duplicated labels like 0.1s.

Why

The previous timeline step selection was tied to grid-count candidates and viewport width. After page load, resizing to a narrower width could make the selected range collapse or shift, so the same report appeared to cover a different time span. The newer direct scale also exposed that maxTime mapped exactly to canvasWidth, while thumbnails use x as their left edge, which could place the final screenshot off-canvas.

This change separates the concerns:

• derive a readable tick interval from the target grid width;
• scale the canvas against a padded visible range;
• keep thumbnail positions and tick labels consistent across widths.

Validation

• ./node_modules/.bin/vitest --run src/components/timeline

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