fix: redirect to role-appropriate dashboard after GitHub OAuth (#77)

[rafaio1]

Fixes #77.

• Replace the hardcoded /dashboard/contributor redirect in CallbackClient with role-aware logic that reads user.roles after login(token) resolves.
• Precedence: maintainer > sponsor > contributor. A user with roles: ["maintainer"] lands on /dashboard/maintainer; ["sponsor"] lands on /dashboard/sponsor; multiple roles follow the same precedence.
• Falls back to /dashboard/contributor when user is null (refresh failed internally) or roles is empty/absent — explicit, intentional default rather than an unconditional assumption.
• Scope note: This fix is scoped to the post-OAuth redirect target only. user.roles remains unused elsewhere in the app (confirmed via grep); Navbar/DashboardShell role-based rendering is covered separately by Fix Navbar loading-state flashes and multi-role rendering logic #48.
• TypeScript compiles cleanly with npx tsc --noEmit.

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