feat(app): add configurable global background wallpaper

apps/desktop/src/stores/__tests__/settingsStore.backgroundImage.spec.ts

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+import { describe, expect, it } from "vitest";
+import { DEFAULT_EDITOR_SETTINGS, normalizeEditorSettings } from "@/stores/settingsStore";
+import { defaultBackgroundImageSettings } from "@/lib/app/appBackgroundImage";
+
+describe("normalizeEditorSettings backgroundImage", () => {
+  it("merges default background image settings for legacy configs missing the field", () => {
+    const normalized = normalizeEditorSettings({ fontFamily: "'Fira Code', monospace" } as never);
+    expect(normalized.backgroundImage).toEqual(defaultBackgroundImageSettings());
+    expect(normalized.fontFamily).toBe("'Fira Code', monospace");
+  });
+
+  it("keeps configured background image values and clamps invalid ones", () => {
+    const normalized = normalizeEditorSettings({
+      backgroundImage: { filePath: "/data/background-image.jpg", fileName: "a.jpg", opacity: 0.4, blur: 6, displayMode: "tile" },
+    } as never);
+    expect(normalized.backgroundImage).toEqual({ filePath: "/data/background-image.jpg", fileName: "a.jpg", opacity: 0.4, blur: 6, displayMode: "tile" });
+
+    const clamped = normalizeEditorSettings({ backgroundImage: { opacity: 12, blur: -1 } } as never);
+    expect(clamped.backgroundImage.opacity).toBe(1);
+    expect(clamped.backgroundImage.blur).toBe(0);
+  });
+
+  it("drops unknown fields from stored settings (showOn* from the removed scoped mode)", () => {
+    const normalized = normalizeEditorSettings({
+      backgroundImage: { filePath: "/data/background-image.jpg", fileName: "a.jpg", opacity: 0.5, blur: 0, showOnWelcome: true, showOnEditor: true },
+    } as never);
+    expect(normalized.backgroundImage).toEqual({ filePath: "/data/background-image.jpg", fileName: "a.jpg", opacity: 0.5, blur: 0, displayMode: "fill" });
+  });
+
+  it("uses default settings for a brand-new store baseline", () => {
+    expect(DEFAULT_EDITOR_SETTINGS.backgroundImage).toEqual(defaultBackgroundImageSettings());
+  });
+});