feat: add 405 method not allowed error handler

[rudra3007-pro]

Summary

Added a 405 Method Not Allowed error handler to match the existing 404
and 500 handlers in app.py. Also added a styled templates/405.html
page consistent with the rest of the error pages in the app.

Related Issue

Closes #326

Type of Change

• Feature — adds new functionality

What Was Changed

File	Change made
app.py	Added method_not_allowed error handler for 405
templates/405.html	Added styled 405 error page

How to Test This PR

1. Clone this branch: git checkout feat/405-error-handler
2. Install dependencies: pip install -r requirements.txt
3. Run the app: python app.py
4. In terminal run:
   curl -X GET http://127.0.0.1:5000/api/recommend
5. You should see the styled 405 page instead of a raw Flask error

Self-Review Checklist

• I have read CONTRIBUTING.md and followed all guidelines
• My branch name follows the convention: feat/405-error-handler
• I have run python tests/test_basic.py and all tests pass
• I have run flake8 . locally and there are no errors
• I have not introduced any print() debug statements
• Every new function I wrote has a docstring
• I have not modified files outside the scope of the linked issue

Notes for Reviewer

Follows the exact same pattern as the existing 404 and 500 handlers.
The 405.html template reuses the same CSS classes as 404.html.

[vercel]

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[komalharshita]

Good contribution overall. The dedicated 405 error handler is implemented cleanly and follows the existing application error-handling pattern well.

The custom template improves user experience compared to the default Flask response, and the PR stays properly scoped without introducing unrelated modifications. Reusing the shared styling system also keeps the implementation consistent with the rest of the project.

Minor non-blocking suggestion: consider moving the inline button styles into the shared stylesheet for better maintainability in future iterations.

Approved for merge.