Wire Auth.js sessions to live GitHub import and portfolio sync

[adambkovacs]

Outcome\nReplace demo-mode reads and writes with signed-in user data flows.\n\n## Scope\n- Persist GitHub OAuth identity and token handling for signed-in users\n- Import each user's starred repositories into Convex\n- Read dashboard, search, repo detail, analytics, and reports from live portfolio data\n- Keep demo-mode as a local fallback only\n\n## Acceptance criteria\n- A signed-in user can import real starred repositories\n- Data is isolated per user\n- Dashboard and drill-down screens render from Convex, not demo fixtures\n- Repeat imports are idempotent