fix(avatars): robust avatar resolution for miners using numeric IDs

[watcharaponthod-code]

Summary

This PR fixes a bug where avatars fail to load in the leaderboard and miner lists when a miner's author name (username) is missing and the system falls back to their numeric githubId.

Previously, the system would attempt to fetch the avatar from github.com/{id}.png, which returns a 404 for numeric IDs. I have refactored the avatar utility to automatically detect numeric identifiers and use the correct avatars.githubusercontent.com/u/{id} endpoint.

Changes

• Utility Refactor (src/utils/avatar.ts): Added getGithubAvatarSrc, a robust resolver that handles both usernames and numeric IDs with auto-detection logic.
• Global Update (src/utils/ExplorerUtils.ts): Re-exported the robust utility to ensure all callers benefit from the fix.
• Leaderboard Fix (LeaderboardSidebar.tsx, MinerCard.tsx, MinersList.tsx): Updated call sites to pass both author and githubId, providing the resolver with the best available data.

Verification

• Verified that numeric strings (e.g., "240883001") now resolve to https://avatars.githubusercontent.com/u/240883001 instead of https://github.com/240883001.png.
• Verified that alphabetic usernames still resolve to the standard github.com/{user}.png URL.
• Ensured no regressions in other components using the shared utility.