Fix report transition and screenshot extension

[aminhanifm]

Summary

• Scroll immediately to the top when sample or uploaded CSV analysis becomes ready.
• Add a Playwright regression that verifies the report heading at desktop and mobile widths after starting from a scrolled landing page.
• Add unit assertions for both analysis entry points.
• Rename the README screenshot to .jpg so its extension matches its JPEG encoding.

Root cause

The landing page scroll position persisted when React switched to the report because the ready-state transition did not reset the document scroll. The screenshot contained JPEG bytes but used a .png filename.

Impact

The report heading is visible as soon as analysis completes on desktop and mobile, and the README now serves the screenshot with the correct file type. Profiling calculations and interface design are unchanged.

Validation

• npm run format
• npm run lint
• npm run typecheck
• npm test with 26 passing tests
• npm run test:e2e with 4 passing Playwright tests
• npm run build