fix: prevent silent goal erasure when loadGoals fails

[just-tanvi]

Description

This PR fixes a critical UX bug where the Goals widget would silently clear all visible goals and revert to the empty state if the /api/goals endpoint returned a non-OK response (e.g. 401 Unauthorized, 500 Internal Server Error).

Previously, loadGoals called .json() without verifying response.ok. Since error responses return an error payload (without a goals array), data.goals would be undefined. The widget defaulted this to an empty array and called setGoals([]), effectively wiping the user's view without giving them any feedback that a network/server issue had occurred.

Changes Made

• Added a !response.ok guard in loadGoals (inside src/components/GoalTracker.tsx) that immediately throws an error containing the HTTP status code.
• Updated the .catch(() => {}) blocks at the three affected call sites to explicitly catch these network errors and call setSyncError(...) with an appropriate, user-facing error message.
• Adjusted test assertions to ensure tests accurately reflect the widget's gracefully handled error states.

Testing

• Verified that all unit tests for the GoalTracker component (npx vitest run test/GoalTracker.test.ts) pass successfully.
• Verified that a failing loadGoals call now preserves the current goals in the UI and correctly bubbles the error up to the UI banner.

closes #2285

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GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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• level:intermediate — 35 pts
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[just-tanvi]

@Priyanshu-byte-coder I will try to fix the 2 failing checks in the pr but it might take me some time, Thankyou for understanding !