Fix day‑1 timezone bug and corresponding heatmap rendering alignment

[ctbfl]

Summary

This PR resolves the day‑1 issue in the contribution graph and then fixes the remaining rendering alignment problems in the weekly/monthly/yearly views.

Approach

1. Found the day‑1 bug and fixed it at the utility layer.
2. After that, rendering was still misaligned, so a second round of fixes targeted the render logic.

Root cause

• Date inputs (especially YYYY‑MM‑DD strings and UTC‑midnight values) were interpreted in local time and could shift to the previous day.
• Some render paths re‑parsed ISO strings with new Date(...), which re‑introduced offsets and broke alignment.

Fixes

• parseDate now treats pure YYYY‑MM‑DD as local midnight using new Date(y, m-1, d).
• Other strings still use new Date(...).
• generateByData uses parseDate to normalize incoming data.
• Fix the rendering part accordingly.

Files touched

• src/util/dateUtils.ts
• src/render/matrixDataGenerator.ts
• src/render/monthTrackGraphRender.ts
• src/render/gitStyleTrackGraphRender.ts
• src/render/graphRender.ts

Testing

• Manual verification on local machine:
  • Weekly, monthly, and yearly views all render correctly with no day‑1 shift.