fix: bucket the badge streak by local day instead of UTC

[Anexus5919]

📌 Related Issue

Fixes #868

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📝 Description

calculateStreak in useBadges.ts bucketed each workout by its UTC date (toISOString().slice(0, 10)) and computed today/yesterday in UTC. For users not at UTC, workouts on the same local day can land in different UTC buckets and consecutive local days can collapse into one, miscounting the badge streak near the local/UTC day boundary.

🔹 What has been changed?

• src/hooks/useBadges.ts: bucket each workout, and today/yesterday, by its local calendar date (${getFullYear()}-${getMonth()+1}-${getDate()}) instead of the UTC date. calculateStreak is now exported so the pure logic can be unit tested.
• src/hooks/__tests__/useBadges.test.ts: adds tests, including the regression case of two workouts on consecutive local days that fall on the same UTC date.

🔹 Why are these changes needed?

• A per-user streak should be measured in the user's local days. UTC bucketing miscounts near midnight for any non-UTC timezone.

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🛠️ Type of Change

• 🐛 Bug Fix

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🧪 Testing

✅ Tests Performed

• Tested locally
• npx vitest run src/hooks/__tests__/useBadges.test.ts -> 3 tests pass.
• Confirmed the new boundary test is a real guard: with the previous UTC bucketing it fails (two consecutive local days are counted as a streak of 1 instead of 2), and it passes with the local-day fix.
• npx tsc --noEmit: useBadges.ts clean; npx eslint: 0 problems.

🌐 Browsers Tested

Not applicable (streak-count logic; verified via unit tests).

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📷 Screenshots / Demo (if applicable)

Not applicable.

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📋 Checklist

• I have read the project's CONTRIBUTING guidelines
• My code follows the project style guidelines
• I have performed a self-review of my code
• I have tested my changes locally
• I have added/updated documentation where necessary (not applicable)
• My changes do not introduce new warnings or errors
• This PR is linked to an existing issue

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💬 Additional Notes

Branched from latest upstream/main (5464425). This is the badge streak in useBadges.ts, separate from the workout streak in streakUtils.ts (#867).

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[Anexus5919]

@Somil450 @diksha78dev Kindly have a review on this pr. Thanks!