fix: resolve double response body read in handleSync

[just-tanvi]

Description

This PR fixes a bug in the handleSync function where the fetch response body was being consumed twice when the API returned an HTTP 429 Too Many Requests status code.

Previously, the first .json() call inside the initial try/catch consumed the response stream, causing a subsequent .json() call in the 429 status check block to throw a TypeError: body stream already read. This swallowed the intended rate limit error message and resulted in a generic "Network error. Failed to sync goals." being shown to the user.

Changes Made

• Refactored handleSync in src/components/GoalTracker.tsx to read the response body exactly once into an errData variable.
• Used the stored errData to properly branch on res.status (429, 401, 502, etc.) without re-reading the stream.
• Removed dead code (the unused msg variable).
• Ensured the fallback error message matches the existing test/GoalTracker.test.ts expectations ("Sync failed. Please try again.").

Testing

• Verified that all unit tests in test/GoalTracker.test.ts pass successfully.
• Verified that TypeScript compilation (tsc --noEmit) passes with zero errors.

closes #2283

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