feat: add code file extensions

[Digvijay557]

Summary

Added support for additional code and configuration file extensions across the backend, frontend, and desktop file picker.

Why

NoRefund previously supported only a small set of file extensions. This expands supported file detection to common programming and configuration files such as JavaScript, TypeScript, Java, C/C++, Go, Rust, HTML, CSS, YAML, TOML, XML, SQL, and shell scripts.

Testing

• pytest passes
• npm test passes (frontend changes)
• ruff check src/ is clean
• Verified manually in a running dev build (UI changes)
• Added/updated a test covering this change

Checklist

• No internal planning/handoff/session-notes files referenced or included
• Bridge changes: frontend/src/lib/types.ts updated to match any changed core/ dataclass fields

[Phantom-VK]

I won't be able to review or accept this PR.
Following are my observations:

• You didn't mention any issue or tagged any issue regarding this PR. (I believe this PR is regarding issue no. 57, 58, 59,please make sure to create one branch for each one issue, and changes related to it only).
• You didn't discussed before if you can take the issue and work on it, please go to related issue and add a comment that "you would like to work on this issue and this is your approach". Then I will assign that issue to you.
• Then you can create proper PR for that particular issue only.
• Please complete the tests given in your PR description. and add screenshots as well, it will help reviewers to check that the implementation is working. Only open PR when you have tested and verified the changes locally.
• Please add proper labels to your PR.
• Also I checked your branch name is: Digvijay557feat/add-code-extensions, please rename it to Digvijay557/feat/add-code-extensions (Branch name may change based on an issue).

These are some of the best practices for open source contribution! :}