feat: add community files and extract proxy port constants

[ykd007]

Description

This PR addresses four community/maintainability issues in one pass:

#433 — Extract hardcoded proxy ports as named constants
Added DefaultForwardProxyPort = ":8080" and DefaultTransparentProxyPort = ":8081" near the top of src/backend/config/config.go. Replaced all hardcoded string literals with these constants (including the fallback in server.go).

#424 — Add CONTRIBUTING.md
New CONTRIBUTING.md at the repo root covering: prerequisites, dev setup, running backend and frontend tests, branching and PR conventions, and code style guidance for both Go and TypeScript.

#425 — Add PR template
New .github/PULL_REQUEST_TEMPLATE.md with sections for description, linked issue (Closes #), testing done, and a pre-submit checklist.

#426 — Add feature request issue template
New .github/ISSUE_TEMPLATE/feature_request.yml with structured fields: problem, proposed solution, alternatives considered, and additional context.

Linked Issues

Closes #433
Closes #424
Closes #425
Closes #426

Testing Done

• Verified Go source compiles with the constant substitutions in place (module-level go build is consistent with pre-existing import path setup).
• All new files are well-formed (YAML validated, Markdown renders correctly).
• No logic changes beyond the constant extraction.

Checklist

• Constants replace all hardcoded :8080/:8081 occurrences in config and server packages
• CONTRIBUTING.md is concise and practical
• PR template includes description, linked issue, testing, and checklist
• Feature request template uses GitHub Forms YAML with required/optional fields
• No unrelated changes included

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[ykd007]

I have read the CLA Document and I hereby sign the CLA

[hanneshapke]

Hi @ykd007
Thank you for your fantastic contribution! Would you mind breaking up the PR in one PR per issue?
That will speed up the review and merge.

Thank you!

[ykd007]

Hi @hanneshapke — split into 4 focused PRs as requested:

• feat: extract hardcoded proxy ports as named constants #439 — extract proxy port constants (closes Extract default proxy ports as named constants in config.go #433)
• feat: add CONTRIBUTING.md #440 — add CONTRIBUTING.md (closes Add CONTRIBUTING.md #424)
• feat: add PR template #441 — add PR template (closes Add a pull request template under .github/ #425)
• feat: add feature request issue template #442 — add feature request issue template (closes Add a feature-request issue template #426)

Closing this combined PR. Thanks for the feedback!