fix(calculate): remove dead-code reassignment breaking streak grace-period check

[PadmakarBagade]

Fixes #7894

Description

calculateStreak() in lib/calculate.ts had a dead-code bug: the grace-period gap check computed the correct todayIndex (either -1 if the gap exceeded grace, or lastIndex otherwise), but an unconditional todayIndex = lastIndex; immediately after the if/else overwrote that result every time. This meant a user's streak was never reset even when their last recorded contribution was older than their grace period — the core streak feature could report an alive streak when it should have shown 0.

This PR removes the redundant line so the grace-period check actually takes effect, and adds a regression test covering both the "gap exceeds grace" (streak resets to 0) and "gap within grace" (streak stays alive) cases.

Type of Change

• Other (Bug fix, refactoring, docs)

Files Changed

• lib/calculate.ts — removed the dead-code todayIndex = lastIndex; line
• lib/calculate.grace-period.test.ts — new regression tests

Testing

• Ran npx vitest run lib/calculate — all calculate-suite tests pass (including the two new tests) alongside my change.
• Ran the full suite; unrelated pre-existing failures observed (not touching lib/calculate.ts or streak logic) were confirmed to fail identically on main before this change, so they are unrelated to this fix.

Checklist

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally.
• I have run formatting checks for the changed files and npm run lint locally.
• My commit follows the Conventional Commits format.
• I have updated README.md if I added a new theme or URL parameter. (N/A — no new theme or URL param)
• I have starred the repo.
• I have made sure that I have only one commit to merge in this PR.

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🚨 Hey @PadmakarBagade, the CI Pipeline is failing on this PR and it has been marked as status:blocked.

Please fix the issues before this can be reviewed. Here's how:

1. Run checks locally before pushing:

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npm run test           # Run unit tests (Vitest)
npm run build          # Verify production build passes

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