UX safety: confirm destructive deletes, scope dice clear, back up sessionLog, add 404

- New themed useConfirm() hook (no native dialogs). World deletes (notes, quests,
  homebrew, maps) now confirm before removing.
- Dice 'Clear history' is disabled with no active campaign (it used to wipe EVERY
  campaign's rolls) and now confirms; only ever clears the active campaign.
- sessionLog is included in backup/restore and the campaign cascade-delete — it was
  silently dropped on backup and orphaned on campaign delete.
- Unknown URLs render a themed 'Page not found' with a link home instead of a blank shell.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/ui/useConfirm.tsx

@@ -0,0 +1,51 @@
+import { useState } from 'react';
+import { Modal } from './Modal';
+import { Button } from './Button';
+
+interface ConfirmOptions {
+  title: string;
+  message: React.ReactNode;
+  confirmLabel?: string;
+  danger?: boolean;
+}
+
+/**
+ * Promise-based confirmation using the themed Modal (no native window.confirm).
+ *
+ *   const { confirm, confirmElement } = useConfirm();
+ *   ...
+ *   <Button onClick={async () => { if (await confirm({ title, message })) doIt(); }} />
+ *   {confirmElement}
+ */
+export function useConfirm() {
+  const [state, setState] = useState<(ConfirmOptions & { resolve: (ok: boolean) => void }) | null>(null);
+
+  const confirm = (opts: ConfirmOptions) =>
+    new Promise<boolean>((resolve) => setState({ ...opts, resolve }));
+
+  const settle = (ok: boolean) => {
+    state?.resolve(ok);
+    setState(null);
+  };
+
+  const confirmElement = state ? (
+    <Modal
+      open
+      onClose={() => settle(false)}
+      title={state.title}
+      className="max-w-md"
+      footer={
+        <>
+          <Button variant="ghost" onClick={() => settle(false)}>Cancel</Button>
+          <Button variant={state.danger === false ? 'primary' : 'danger'} onClick={() => settle(true)}>
+            {state.confirmLabel ?? 'Delete'}
+          </Button>
+        </>
+      }
+    >
+      <div className="text-sm text-muted">{state.message}</div>
+    </Modal>
+  ) : null;
+
+  return { confirm, confirmElement };
+}