feat: infinite scroll, configurable commit count, and performance improvements

[Sagargupta16]

Summary

This PR addresses three related enhancement requests by improving the commit loading experience:

• Infinite scroll (Autoload commits when we scroll down rather than having a "load more" button #93) - Replaces the "Load More" button with automatic loading via IntersectionObserver as the user scrolls down. The button is kept as a fallback.
• Configurable commits per page (Allow to configure a bigger number of commits loaded by default #92) - Adds a settings dropdown in the extension popup (10/20/30/50/100). Default changed from 10 to 30 for a more useful initial view. Preference persists via chrome.storage.sync.
• Performance improvement for large repos (Loading too slow if too many branches #41) - The configurable page size and deduplicated query builder reduce unnecessary API calls. Repos with many branches benefit from fetching more commits per branch upfront rather than paginating through small batches.

Changes

File	What changed
manifest.json	Added storage permission, added getCommitsPerPage.js to content scripts
js/getCommitsPerPage.js	New - helper to read user's commits-per-page preference
js/popup.js	Settings load/save for commits-per-page dropdown
html/popup.html	Added settings UI section below the demo image
js/fetchCommits.js	Deduplicated query into buildBranchQuery(), uses configurable page size
js/fetchFilteredCommits.js	Uses configurable page size in filtered queries
js/sortCommits.js	Uses configurable page size for initial slice
js/fetchFurther.js	Uses configurable page size for pagination threshold and slice
js/showCommits.js	Infinite scroll via IntersectionObserver in addNextPageButton(), configurable page size

How to test

1. Load the extension unpacked in Chrome (chrome://extensions > Load unpacked)
2. Visit any GitHub repo and click the Commits tab
3. Scroll down - commits should auto-load without clicking "Load More"
4. Click the extension icon - change "Commits per page" to 50
5. Reload the page and click Commits - should show 50 commits initially
6. Test on a repo with many branches (e.g., flutter/flutter) - should load faster with larger page size

Screenshots

Settings in popup:

The dropdown appears below the existing demo image with options 10/20/30/50/100.

Closes #93, Closes #92, Addresses #41