feat: display GitHub API rate limit in sidebar

[Flo0806]

Summary

Show the current GitHub API rate limit usage in the sidebar as a compact progress bar with reset timer.

Details

• Fetch via GET /rate_limit or read x-ratelimit-* headers from existing responses
• Display in sidebar: small progress bar showing remaining/total requests
• Show reset time (e.g. "resets in 42min")
• Visual warning when approaching limit (e.g. < 20% remaining)

Why

As a maintainer using Flumen with polling and multiple repos, it's important to know how much API quota is left to avoid hitting rate limits unexpectedly.