feat: enhanced dashboard with personality, velocity, timeline, and comparison

[Sagargupta16]

Summary

• Coding personality badge - Categorizes users as Builder, Reviewer, Collaborator, Maker, or All-Rounder based on their contribution mix
• Quick insights row - Shows avg contributions per active day, velocity trend (up/down/neutral comparing last 4 vs previous 4 weeks), and own/fork repo ratio
• Repo growth timeline - Bar chart showing repository creation history by year
• Profile comparison - When viewing someone else's profile, shows a side-by-side comparison of contributions, stars, repos, and merged PRs with +/- diff
• UI improvements - Heatmap legend, weekday chart highlight, enlarged donut chart, stat card tooltips, error state with retry button, "<1%" for small percentages, locale-formatted numbers

Changes

• charts.js - Added computePersonality, computeVelocity, computeAvgPerDay, renderRepoTimeline functions
• dashboard.js - New panel sections for personality, insights row, repo timeline, and profile comparison
• content.js - Saves viewer stats on own profile, passes comparison data when viewing others
• storage.js - Added saveViewerStats/getViewerStats for cross-profile comparison persistence
• insights.css - Styles for all new sections plus dark theme support
• api.js - Added createdAt to repo query, removed unused organizations query
• README.md - Updated features list
• CHANGELOG.md - Added v1.1.0 entry

Test plan

• Load extension from dist/, visit your own GitHub profile - verify personality badge, quick stats row, repo timeline appear
• Visit a different user's profile - verify "vs. You" comparison section appears with correct diffs
• Toggle GitHub dark/light theme - verify all new sections adapt correctly
• Check stat card tooltips show exact values on hover
• Verify error state with retry button works when API fails
• Test on profiles with zero contributions (edge case for personality/velocity)