Add ErrorBoundary component and PlanGate test coverage

[n3-rd]

PlanGate is the only component under space/ without tests; every sibling (SpacePicker, SpaceCreator, SpaceList, SpaceEnsurer, FileViewer, SharingTool, ImportSpace) already has a spec file. A test suite for PlanGate would close that gap.

Separately, there's no way to recover from render-time errors. Async errors are caught in try/catch callbacks, but a thrown error during render takes down the whole React tree — which for an embeddable library means crashing the host app. A StorachaErrorBoundary component would let consumers wrap subtrees, show fallback UI, and optionally report errors.

Scope: new test file for PlanGate, new ErrorBoundary component + tests, export wiring in index.ts.

[n3-rd]

Hello, should I work on this?

[alikhere]

Hello, should I work on this?

@n3-rd
The PlanGate test coverage part was already implemented in #23 last week.

The remaining work here is adding the ErrorBoundary component and its tests. If you'd like to work on that part, please update the issue title and desc (for ex: "Add ErrorBoundary component and tests") and open a PR

Also, don't forget to run pnpm run ci after the implementation to ensure the CI workflow passes.