fix(ui): harden theme toggle accessibility

src/lib/ui/theme.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, test } from "vitest";
 import {
   getNextUiTheme,
   isUiTheme,
+  persistUiTheme,
   resolveInitialUiTheme,
   UI_THEME_STORAGE_KEY,
 } from "./theme";
@@ -25,6 +26,26 @@ describe("UI theme preference", () => {
     expect(resolveInitialUiTheme(storage)).toBe("dark");
   });
 
+  test("falls back when stored preferences cannot be read", () => {
+    const storage = {
+      getItem() {
+        throw new Error("storage blocked");
+      },
+    };
+
+    expect(resolveInitialUiTheme(storage)).toBe("dark");
+  });
+
+  test("ignores persistence failures", () => {
+    const storage = {
+      setItem() {
+        throw new Error("quota exceeded");
+      },
+    };
+
+    expect(() => persistUiTheme("light", storage)).not.toThrow();
+  });
+
   test("detects and toggles supported themes", () => {
     expect(isUiTheme("light")).toBe(true);
     expect(isUiTheme("dark")).toBe(true);