fix: resolve all QA report issues from 2026-06-05 sweep

Merges 77 QA issue fixes across routing, editor, i18n, pipeline,
and settings. Includes catch-all 404 page, keyboard shortcuts,
export filename, password validation, selection masking, brush flow,
eraser modes, feather control, pipeline persistence, RTL properties,
license info, and i18n for all 21 locales.

apps/web/src/pages/not-found-page.tsx

@@ -0,0 +1,22 @@
+import { Link } from "react-router-dom";
+import { useTranslation } from "@/contexts/i18n-context";
+
+export function NotFoundPage() {
+  const { t } = useTranslation();
+
+  return (
+    <div className="flex h-screen items-center justify-center bg-background text-foreground">
+      <div className="text-center space-y-4 max-w-md px-6">
+        <h1 className="text-6xl font-bold text-primary">404</h1>
+        <h2 className="text-xl font-semibold">{t.common.pageNotFound}</h2>
+        <p className="text-sm text-muted-foreground">{t.common.pageNotFoundDescription}</p>
+        <Link
+          to="/"
+          className="inline-block px-4 py-2 rounded-lg bg-primary text-primary-foreground text-sm font-medium"
+        >
+          {t.common.goHome}
+        </Link>
+      </div>
+    </div>
+  );
+}