[BUG]: Missing pagination safety guard in fetchContributors() and fetchIssues()

Bug Description

Both fetchContributors() and fetchIssues() iterate through paginated GitHub API responses using an unbounded loop:

js :
for (let page = 1; ; page++) { ... }

Steps to Reproduce :

1. Open services/github.js.
2. Navigate to fetchContributors() and fetchIssues().
3. Observe that both functions use an unbounded pagination loop without a maximum page limit or response validation.

Environment Details :

- OS: Windows 11
- Browser: Brave
- Node.js: 22.18.0

Impact :

Medium - Feature works but has issues

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

Start in services/github.js and inspect fetchContributors() and fetchIssues(), focusing on their paginated GitHub API loops and response handling. The issue is complete when both functions have a defined pagination safety limit and validate responses instead of iterating unboundedly.

索引モデルが issue の本文から書いたものです。