Test(error): add empty-fallback checks and tests for root error boundaries

[diksha78dev]

Description

Fixes #4200

This PR adds empty-fallback checks and robust test coverage for the root-level error boundaries (app/error.tsx and app/global-error.tsx).

Previously, receiving null, undefined, or malformed error objects would cause a secondary crash inside the boundaries due to direct access to error.message, preventing the styled error UI from rendering.

Changes Made:

• Safely extracted the error message using const errorMessage = error?.message || '' before passing it to console logs, clipboard text, or rendering.
• Cleaned up an unused Link import warning in app/global-error.tsx.
• Created app/error.empty-fallback.test.tsx and app/global-error.empty-fallback.test.tsx to verify resilient boundary behavior under multiple malformed/missing error object shapes.

Pillar

• 🎨 Pillar 1 — New Theme Design
• 📐 Pillar 2 — Geometric SVG Improvement
• 🕐 Pillar 3 — Timezone Logic Optimization
• 🛠️ Other (Bug fix, refactoring, docs)

Visual Preview

(N/A — This is a codebase resilience and unit testing PR. No change is made to standard UI designs or visual layouts).

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (npx vitest run app/error.empty-fallback.test.tsx app/global-error.empty-fallback.test.tsx).
• I have run npm run format and npm run lint locally and resolved all errors (CI will fail otherwise).
• My commits follow the Conventional Commits format (e.g., feat(themes): ..., fix(calculate): ...).
• I have updated README.md if I added a new theme or URL parameter.
• I have starred the repo.
• I have made sure that i have only one commit to merge in this PR.
• The SVG output matches the CommitPulse "premium quality" aesthetic standard (no raw elements, smooth animations, correct fonts).
• (Recommended) I joined the CommitPulse Discord community for contributor discussions, mentorship, and PR support.

[Aamod-Dev]

Thanks for the contribution. I went through the changes and the overall approach looks good.

[github-actions]

🎉 Congratulations @diksha78dev! Your PR has been successfully merged. 🚀

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