fix(ui): restore ThemeSelector and add error resilience testing

[ThePriyanuj]

Description

Fixes #6888

This PR addresses the issue where ThemeSelector was causing an "Element type is invalid: expected a string... but got: undefined" crash. The component file was accidentally overwritten during previous testing. This PR restores the component implementation and adds a dedicated error resilience test suite (ThemeSelector.error-resilience.test.tsx) to guarantee the component renders gracefully even with invalid props, undefined themes, or unhandled child exceptions. It also resolves a merge conflict in route.ts.

Pillar

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

Visual Preview

(No visual changes, fixes a crash)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally (localhost:3000/api/streak?user=YOUR_USERNAME).
• 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 faster PR support.

[github-actions]

🚨 Hey @ThePriyanuj, the CI Pipeline is failing on this PR and it has been marked as status:blocked.

Please fix the issues before this can be reviewed. Here's how:

1. Run checks locally before pushing:

npm run format:check   # Check Prettier formatting
npm run lint           # Run ESLint
npm run typecheck      # TypeScript type check
npm run test           # Run unit tests (Vitest)
npm run build          # Verify production build passes

2. Auto-fix common issues:

npm run format         # Auto-fix formatting with Prettier
npm run lint -- --fix  # Auto-fix lint errors where possible

3. Check the full failure log here:
👉 View CI Run

Once you push a fix and the CI passes, the status:blocked label will be removed automatically. 💪

[ThePriyanuj]

/unclaim

[Aamod007]

The CI checks (Format / Lint / Typecheck / Test) are currently failing and the PR is marked as blocked. Please fix the failing checks so we can proceed with the review.

[github-actions]

⚠️ Hey @ThePriyanuj, this pull request has been inactive for 3 days. It will be automatically closed in 2 days if no further activity occurs.

If you are still working on this, please push your latest changes or leave a comment to keep it active.