[YashBora21]

Fix SPRINTER badge awarding multi-day progress

SPRINTER compared the last two solved-count snapshots without checking their dates, so +7 solves spread over several days could still earn it.

Now it only awards the badge when the two snapshots are on consecutive calendar days and the increase is 7 or more.

Tests (node --test tests/badges.test.cjs):

• Consecutive days, +7: earned
• Consecutive days, +6: locked
• 5-day gap, +7: locked
• Single entry: locked

Also checked manually on the local profile page and ran Prettier.

Fixes #544

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[jagdish-15]

Thanks for adding the test file!