fix(02): CSS-safe occurrence ids + error boundary to surface render errors

Schedule-X rejects ids containing ':' '[' ']' (the old ${uid}::${iso} form) — mint ev-<uid>-<epochMs> instead. Add an ErrorBoundary so a render throw shows the error instead of a blank page.

apps/pwa/src/components/ErrorBoundary.tsx

@@ -0,0 +1,69 @@
+/**
+ * ErrorBoundary — catches render/runtime errors in the calendar tree and shows a
+ * readable fallback instead of a blank white page.
+ *
+ * A white-screen-on-error calendar is unacceptable UX (the non-technical Apple member
+ * would just see nothing). This boundary degrades gracefully and surfaces the error
+ * text + component stack so failures are diagnosable in the field.
+ */
+
+import React from 'react'
+
+interface ErrorBoundaryProps {
+  children: React.ReactNode
+}
+
+interface ErrorBoundaryState {
+  error: Error | null
+  info: string | null
+}
+
+export class ErrorBoundary extends React.Component<ErrorBoundaryProps, ErrorBoundaryState> {
+  state: ErrorBoundaryState = { error: null, info: null }
+
+  static getDerivedStateFromError(error: Error): Partial<ErrorBoundaryState> {
+    return { error }
+  }
+
+  componentDidCatch(error: Error, info: React.ErrorInfo) {
+    // Log for DevTools / future telemetry; also kept in state for on-screen display.
+    console.error('[CalendarShell crash]', error, info.componentStack)
+    this.setState({ info: info.componentStack ?? null })
+  }
+
+  render() {
+    if (this.state.error) {
+      return (
+        <div
+          role="alert"
+          style={{
+            padding: '24px',
+            fontFamily: 'system-ui, sans-serif',
+            color: '#7f1d1d',
+            background: '#fef2f2',
+            height: '100dvh',
+            overflow: 'auto',
+            boxSizing: 'border-box',
+          }}
+        >
+          <h2 style={{ marginTop: 0 }}>The calendar hit an error</h2>
+          <p style={{ fontWeight: 600 }}>{String(this.state.error?.message ?? this.state.error)}</p>
+          <pre
+            style={{
+              whiteSpace: 'pre-wrap',
+              fontSize: '12px',
+              background: '#fff',
+              border: '1px solid #fecaca',
+              borderRadius: '6px',
+              padding: '12px',
+            }}
+          >
+            {this.state.error?.stack}
+            {this.state.info ? '\n\n--- component stack ---' + this.state.info : ''}
+          </pre>
+        </div>
+      )
+    }
+    return this.props.children
+  }
+}

apps/pwa/src/main.tsx

@@ -12,6 +12,7 @@ import React from 'react'
 import ReactDOM from 'react-dom/client'
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
 import App from './App.js'
+import { ErrorBoundary } from './components/ErrorBoundary.js'
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -25,7 +26,9 @@ const queryClient = new QueryClient({
 ReactDOM.createRoot(document.getElementById('root')!).render(
   <React.StrictMode>
     <QueryClientProvider client={queryClient}>
-      <App />
+      <ErrorBoundary>
+        <App />
+      </ErrorBoundary>
     </QueryClientProvider>
   </React.StrictMode>,
 )