Fix About-section injection and default-branch detection

[DervexDev]

This PR cherry-picks two specific fixes from PR #82 by @dongguacute while preserving the existing custom ghloc backend architecture.

Changes Included

1. Fix GitHub About Section Injection

• More resilient DOM detection: Uses normalize-space(.)="About" XPath instead of text()="About" to handle varying whitespace in GitHub's DOM
• MutationObserver support: Waits for the About section if it's not present on initial page load (handles dynamic content loading)
• Prevent widget duplication: Reuses the existing #github-loc element on re-injection instead of creating duplicates

2. Fix Default Branch Detection

• Smart branch detection: Checks tree/blob URL path first, then falls back to branch selector UI elements
• Stop assuming "main": Returns empty string when no branch is detected, allowing the backend to resolve the repository's actual default branch
• Proper URL encoding: Encodes branch parameter in Stat link and makes it optional when empty

Changes NOT Included

The following changes from PR #82 were intentionally left out to preserve the existing architecture:

• ❌ Public API host swap (ghloc.dev / ghloc.ifels.dev)
• ❌ fetchLocalLoc / fetchPublicLoc split
• ❌ In-browser GitHub tree+blob counting
• ❌ isPublic parameter driving API choice
• ❌ Optional Ghloc-Authorization header

Why This Approach?

The custom ghloc backend (ghloc-api.vercel.app / ghloc.vercel.app) already supports private repositories via the user's GitHub token with the salt-based authentication flow. Switching to the public API would break this functionality.

These cherry-picked fixes address real issues:

• The About section injection was failing on some GitHub pages due to DOM changes
• The extension was getting 404 errors on repositories with default branches other than "main"

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Credits: DOM and branch detection improvements extracted from PR #82 by @dongguacute

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