fix: add missing 500 error template

[ionfwsrijan]

Summary [required]

This PR fixes the registered internal server error handler by adding the missing templates/500.html page. Without this template, Flask can raise a secondary TemplateNotFound: 500.html error when handling an unexpected server error. The new page follows the same structure as the existing 404.html template and includes a regression test for the 500 handler.

Related Issue [required]

Closes #66

Type of Change [required]

• Bug fix - resolves a broken behaviour
• Feature - adds new functionality
• Data - adds new projects to data/projects.json
• Documentation - updates docs, README, or code comments only
• Style - CSS or visual changes only, no logic change
• Refactor - restructures code without changing behaviour
• Test - adds or updates tests

What Was Changed [required]

File	Change made
templates/500.html	Added a friendly internal server error page matching the existing error-page structure.
tests/test_basic.py	Added a regression test confirming the 500 handler renders the template and returns status 500.

How to Test This PR [required]

1. Clone this branch: git checkout fix/missing-500-template
2. Install dependencies: pip install -r requirements.txt
3. Run the tests: python tests/test_basic.py
4. Optionally run the app with python app.py and trigger an unhandled server-side exception locally to confirm the friendly 500 page renders.

Expected test output:

28 passed, 0 failed out of 28 tests

Test Results [required]

  PASS  test_projects_json_loads
  PASS  test_each_project_has_required_fields
  PASS  test_find_project_by_id_found
  PASS  test_find_project_by_id_missing
  PASS  test_parse_skills_basic
  PASS  test_parse_skills_empty_string
  PASS  test_parse_skills_single_entry
  PASS  test_score_single_project_full_match
  PASS  test_score_single_project_no_match
  PASS  test_get_recommendations_returns_results
  PASS  test_get_recommendations_max_three
  PASS  test_get_recommendations_no_match_returns_empty
  PASS  test_get_recommendations_result_format
  PASS  test_validate_all_valid
  PASS  test_validate_missing_skills
  PASS  test_validate_missing_level
  PASS  test_validate_missing_interest
  PASS  test_validate_missing_time
  PASS  test_validate_all_missing
  PASS  test_home_route
  PASS  test_recommend_api_valid
  PASS  test_recommend_api_missing_field
  PASS  test_recommend_api_empty_body
  PASS  test_project_detail_found
  PASS  test_project_detail_not_found
  PASS  test_internal_server_error_page
  PASS  test_view_code_found
  PASS  test_download_code_found

28 passed, 0 failed out of 28 tests

Screenshots (if UI change)

No screenshot attached. This adds the missing error template and verifies it through the regression test.

Self-Review Checklist [required]

• I have read CONTRIBUTING.md and followed all guidelines
• My branch name follows the convention: feat/, fix/, docs/, data/, style/, test/
• I have run python tests/test_basic.py and all 27 tests pass
• I have run flake8 . locally and there are no errors
• I have not introduced any print() or console.log() debug statements
• Every new function I wrote has a docstring
• I have not modified files outside the scope of the linked issue
• If I changed the UI, I tested it at 375px (mobile) and 1280px (desktop)
• If I added a project to the dataset, it has all required JSON fields

Notes for Reviewer

python -m flake8 . currently reports pre-existing lint issues across unrelated files such as routes/main_routes.py, starter_code/*, and existing tests. I did not modify those files to keep this PR scoped to issue #66.

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[ionfwsrijan]

@komalharshita Kindly review and merge

[komalharshita]

Thanks for addressing the requested changes.

This is a solid bug-fix PR overall. The issue is legitimate, the implementation is properly scoped, and the added regression test is especially valuable because it prevents the 500.html missing-template issue from reappearing in future changes.

I also appreciate that the PR stays focused only on the relevant files without introducing unrelated modifications.

The implementation and test coverage look good to merge. Nice work!