[Web UI] Add /dashboard/github-insights page for GitHub Activity Intelligence

[Midoriya-w]

Sub-issue of #132

Implements the /dashboard/github-insights SvelteKit page consuming the existing /api/analytics/github-insights endpoint.

Changes:

• Stats grid: repos, stars, forks, followers, following, primary language
• AI summary card (hidden when aiSummary is null)
• Language breakdown with visual stacked bar
• Top repositories grid
• Loading, error, and requires-connect states
• Mobile responsive layout

PR: #163

[Midoriya-w]

Hey @ShantKhatri @Harxhit This PR closes sub-issue #322 (which is a sub-issue of #132). The scope is clearly separated this covers only the Web UI piece (/dashboard/github-insights SvelteKit page). Ready for final review!