feat: build DashboardNav component for authenticated layout

[basanth-p]

Summary

Build the DashboardNav component to replace the placeholder header in app/(dashboard)/layout.tsx.

Design spec

• Left: Patch ID logo + wordmark
• Centre: nav links — Dashboard, Score, Settings
• Right: user avatar (from GitHub OAuth metadata) + dropdown (Profile, Sign out)

Acceptance criteria

• Component renders correctly on /dashboard, /dashboard/score, /dashboard/settings
• User avatar pulls from user.user_metadata.avatar_url
• Sign out clears Supabase session and redirects to /
• Responsive — collapses to hamburger on mobile
• Replace TODO comment in app/(dashboard)/layout.tsx