feat: sanitize raw Gemini API error traces (issue #139)

[Talhaasif7]

📌 Related Issue

Fixes #139

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Description of Changes

Sanitized low-level exceptions and credential errors from the Gemini API to prevent raw logs and keys (like API_KEY_INVALID) from leaking to the frontend UI.

• Backend Sanitization: Added a sanitizeError helper function in backend/src/ai/gemini.js that intercepts low-level SDK traces, API key configuration errors, and timeout errors, mapping them to clean, user-friendly statements.
• Frontend Guarding: Added a defensive getCleanInsightError mapper in src/pages/Dashboard.tsx to double-guard raw strings before displaying them in the AI Note card.

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Type of Change

• 🐛 Bug Fix
• ✨ New Feature
• 🎨 UI/UX or Design Update
• Other

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✅ Checklist

• I’ve read the CONTRIBUTING.md guidelines.
• My code follows the project’s conventions.
• I’ve linked the related issue correctly.
• I’ve tested my changes locally.
• I’ve added or updated documentation/comments if needed.
• My PR title follows the branch & commit naming conventions.
• My changes introduce no new warnings or errors.
• I’ve performed a self-review of my work.

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💬 Additional Notes (Optional)

Unit tests ran and passed successfully. Production build (npm run build) and linting (npm run lint) check out with 0 errors.

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