Replace ErrorBoundary fallback with Fluent UI styles and dialog compliance

Docs/Tasks.md

@@ -315,6 +315,8 @@ Reference: `Docs/Refactoring.md` for full analysis of each issue.
 
 **Docs changes needed:** None.
 
+**Status:** Completed.
+
 **Why this is needed:** The architecture explicitly forbids building custom modal overlays ("Use Fluent UI `<Dialog>` for modals and confirmations — never build custom modal overlays"). Hand-built modals bypass Fluent UI's accessibility tree (focus trap, ARIA roles, portal rendering) and duplicate boilerplate that the library provides. A Fluent `<Dialog>` is keyboard-accessible, screen-reader-compatible, and inherits the theme automatically.
 
 ---

frontend/src/components/ErrorBoundary.tsx

@@ -4,6 +4,7 @@
  * Catches render-time exceptions in child components and shows a fallback UI.
  */
 import React from "react";
+import { Button, makeStyles, Text, tokens } from "@fluentui/react-components";
 
 interface ErrorBoundaryState {
   hasError: boolean;
@@ -14,11 +15,49 @@ interface ErrorBoundaryProps {
   children: React.ReactNode;
 }
 
+interface ErrorBoundaryFallbackProps {
+  message: string;
+  onReload: () => void;
+}
+
+const useFallbackStyles = makeStyles({
+  root: {
+    display: "flex",
+    flexDirection: "column",
+    alignItems: "center",
+    justifyContent: "center",
+    minHeight: "100vh",
+    padding: tokens.spacingVerticalL,
+    textAlign: "center",
+    gap: tokens.spacingVerticalM,
+  },
+  message: {
+    maxWidth: "40rem",
+  },
+});
+
+function ErrorBoundaryFallback({ message, onReload }: ErrorBoundaryFallbackProps): React.ReactElement {
+  const styles = useFallbackStyles();
+
+  return (
+    <div className={styles.root} role="alert">
+      <Text as="h1" size={700} weight="semibold">
+        Something went wrong
+      </Text>
+      <Text size={300} className={styles.message}>
+        {message}
+      </Text>
+      <Button appearance="primary" onClick={onReload}>
+        Reload
+      </Button>
+    </div>
+  );
+}
+
 export class ErrorBoundary extends React.Component<ErrorBoundaryProps, ErrorBoundaryState> {
   constructor(props: ErrorBoundaryProps) {
     super(props);
     this.state = { hasError: false, errorMessage: null };
-    this.handleReload = this.handleReload.bind(this);
   }
 
   static getDerivedStateFromError(error: Error): ErrorBoundaryState {
@@ -29,31 +68,17 @@ export class ErrorBoundary extends React.Component<ErrorBoundaryProps, ErrorBoun
     console.error("ErrorBoundary caught an error", { error, errorInfo });
   }
 
-  handleReload(): void {
+  handleReload = (): void => {
     window.location.reload();
-  }
+  };
 
   render(): React.ReactNode {
     if (this.state.hasError) {
       return (
-        <div
+        <ErrorBoundaryFallback
-          style={{
+          message={this.state.errorMessage ?? "Please try reloading the page."}
-            display: "flex",
+          onReload={this.handleReload}
-            flexDirection: "column",
+        />
-            alignItems: "center",
-            justifyContent: "center",
-            minHeight: "100vh",
-            padding: "24px",
-            textAlign: "center",
-          }}
-          role="alert"
-        >
-          <h1>Something went wrong</h1>
-          <p>{this.state.errorMessage ?? "Please try reloading the page."}</p>
-          <button type="button" onClick={this.handleReload} style={{ marginTop: "16px" }}>
-            Reload
-          </button>
-        </div>
       );
     }