fix(website): Validate GitHub repository URLs

[MathurAditya724]

The repository form now parses submitted URLs and accepts URL input only when its hostname is exactly github.com. Existing owner/repo shorthand and canonical GitHub URLs continue to work, while embedded and lookalike hosts are rejected.

Regression coverage includes the bypass variants reported by CodeQL alert #2.

[github-actions]

Codecov Results 📊

✅ 212 passed | Total: 212 | Pass Rate: 100% | Execution Time: 372ms

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +7
Passed Tests	📈 +7
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 87.50%. Project has 684 uncovered lines.
✅ Project coverage is 61.6%. Comparing base (d9789bf) to head (a3d929a).

Files with missing lines (1)

File	Patch %	Lines
website/src/lib/repository-url.ts	87.50%	⚠️ 2 Missing and 2 partials

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    61.37%    61.60%    +0.23%
==========================================
  Files           24        25        +1
  Lines         1761      1777       +16
  Branches      1269      1281       +12
==========================================
+ Hits          1079      1093       +14
- Misses         682       684        +2
- Partials       105       107        +2

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Generated by Codecov Action