feat(web): add app/not-found.tsx and app/error.tsx error pages

[GRACENOBLE]

Problem

Next.js serves its default (unbranded) error pages when a route is not found or an unhandled error is thrown. A project starter should ship with branded error pages from day one.

Acceptance criteria

• app/not-found.tsx — 404 page with a link back to /
• app/error.tsx — client error boundary with a retry button (must be a Client Component with "use client")
• app/global-error.tsx — root-level error boundary for layout errors
• Pages use the existing theme (Tailwind v4, shadcn/ui tokens)

[GRACENOBLE]

Implemented and merged via PR #65.