[Agent] Add error.tsx and loading.tsx route boundaries

Agent brief

Category: ux | Severity: high | Effort: M

Problem

No error.tsx or loading.tsx anywhere under src/app/. Server errors show generic Next.js page; navigations have no skeleton feedback.

Repro

1. Trigger server error on any project route
2. Observe bare Next error boundary
3. Navigate between heavy pages — blank flash

Files

• Add src/app/[projectSlug]/error.tsx, loading.tsx
• Add src/app/admin/error.tsx, loading.tsx

Acceptance criteria

• Branded error UI with retry + home link
• Loading skeletons on feedback, roadmap, docs, changelog

Suggested approach

Start with [projectSlug] segment; reuse PageHeader + Card patterns.