Fix language bar hover tooltip

[CodecCods]

Summary

• Add a custom tooltip for each language bar showing the language name and percentage on hover.
• Keep the native title text while adding keyboard focus support through tabIndex and aria-label.
• Mark language bars as non-drag regions so hover/focus interactions work inside the frameless Electron widget.

Testing

• npm run typecheck
• npx eslint src/renderer/src/components/GitHubStatsWidget.tsx --quiet
• npx prettier --check src/renderer/src/components/GitHubStatsWidget.tsx src/renderer/src/assets/main.css
• npm run build

Note: full npm run lint still fails on pre-existing any errors in src/renderer/src/components/ContributionGrid.tsx.

Closes #8.