Add error boundaries so one render throw doesn't blank the whole app

[lucastononro]

Problem. There is no app/error.tsx, app/global-error.tsx, or any ErrorBoundary in src/.

Why it matters. Given untyped SSE payloads and live-rendered user/agent-authored markdown + self-contained HTML artifacts, one bad payload throwing in render takes down the entire SPA with a blank screen.

Suggested fix. Add a route-level src/app/error.tsx and a src/app/global-error.tsx with reset, plus a component-level boundary around the canvas/markdown renderers.

[lucastononro]

Addressed by #159, now merged into integration branch staging-v0.0.5 (ledger: STAGING-v0.0.5.md). Will land on main via the staging merge — closing.