Add error boundaries (route-level + global)

## Goal
Add a route-level error component for TanStack Router and a global `ErrorBoundary` fallback for unexpected render errors. Trigger one on purpose to demonstrate.

## Acceptance criteria
- [ ] Each major route exports an `errorComponent`
- [ ] A global `ErrorBoundary` wraps the app root
- [ ] Throwing inside a child component shows the fallback UI
- [ ] Reset button recovers without a full page reload

Locate the app root, the major TanStack Router routes, and the child component used to trigger an error. Read how routes and the root are currently composed, then intentionally trigger the child error to verify the fallback and reset behavior. Done means every major route has an error component, the root has a global ErrorBoundary, and recovery works without a full page reload.

Written by the indexing model from the issue text.