clean review nits: typed prefix sentinel, instanceof narrowing, drop dead re-export

- Replace anonymous Error in resolveImageWallpaperUrl with typed
  UnsafeImagePrefixError, mirroring UnsafeAssetPathError so cause
  chains stay discriminable.
- Replace `(err as BackgroundLoadError).cause` casts in wallpaper
  tests with instanceof narrowing (no `as` per project rules).
- Remove unused `WALLPAPER_PATHS` re-export from projectPersistence;
  consumers import directly from @/lib/wallpaper (SSOT).

src/components/video-editor/projectPersistence.ts

@@ -56,8 +56,6 @@ function normalizeWallpaperValue(value: string): string {
 	return CANONICAL_WALLPAPERS.has(canonical) ? canonical : DEFAULT_WALLPAPER;
 }
 
-export { WALLPAPER_PATHS };
-
 export const PROJECT_VERSION = 2;
 
 export interface ProjectEditorState {

src/lib/wallpaper.test.ts

@@ -5,6 +5,7 @@ import {
 	classifyWallpaper,
 	DEFAULT_WALLPAPER,
 	resolveImageWallpaperUrl,
+	UnsafeImagePrefixError,
 	WALLPAPER_COUNT,
 	WALLPAPER_PATHS,
 } from "./wallpaper";
@@ -170,8 +171,14 @@ describe("resolveImageWallpaperUrl", () => {
 		expect(resolveImageWallpaperUrl("/wallpapers/my image.jpg")).toBe("/wallpapers/my%20image.jpg");
 	});
 
-	it("rejects image paths outside /wallpapers/", () => {
-		expect(() => resolveImageWallpaperUrl("/etc/passwd")).toThrow(BackgroundLoadError);
+	it("rejects image paths outside /wallpapers/ with UnsafeImagePrefixError as cause", () => {
+		try {
+			resolveImageWallpaperUrl("/etc/passwd");
+			expect.fail("should have thrown");
+		} catch (err) {
+			if (!(err instanceof BackgroundLoadError)) throw err;
+			expect(err.cause).toBeInstanceOf(UnsafeImagePrefixError);
+		}
 	});
 
 	it("wraps traversal attempts in BackgroundLoadError (preserves UnsafeAssetPathError as cause)", () => {
@@ -179,8 +186,8 @@ describe("resolveImageWallpaperUrl", () => {
 			resolveImageWallpaperUrl("/wallpapers/../etc/passwd");
 			expect.fail("should have thrown");
 		} catch (err) {
-			expect(err).toBeInstanceOf(BackgroundLoadError);
-			expect((err as BackgroundLoadError).cause).toBeInstanceOf(UnsafeAssetPathError);
+			if (!(err instanceof BackgroundLoadError)) throw err;
+			expect(err.cause).toBeInstanceOf(UnsafeAssetPathError);
 		}
 	});
 
@@ -189,8 +196,8 @@ describe("resolveImageWallpaperUrl", () => {
 			resolveImageWallpaperUrl("/wallpapers/%2e%2e/app.asar");
 			expect.fail("should have thrown");
 		} catch (err) {
-			expect(err).toBeInstanceOf(BackgroundLoadError);
-			expect((err as BackgroundLoadError).cause).toBeInstanceOf(UnsafeAssetPathError);
+			if (!(err instanceof BackgroundLoadError)) throw err;
+			expect(err.cause).toBeInstanceOf(UnsafeAssetPathError);
 		}
 	});
 
@@ -226,8 +233,8 @@ describe("resolveImageWallpaperUrl", () => {
 			resolveImageWallpaperUrl("/wallpapers/wallpaper1.jpg");
 			expect.fail("should have thrown");
 		} catch (err) {
-			expect(err).toBeInstanceOf(BackgroundLoadError);
-			expect((err as BackgroundLoadError).cause).toBeInstanceOf(AssetBaseUnavailableError);
+			if (!(err instanceof BackgroundLoadError)) throw err;
+			expect(err.cause).toBeInstanceOf(AssetBaseUnavailableError);
 		}
 	});
 });

src/lib/wallpaper.ts

@@ -37,6 +37,13 @@ export function classifyWallpaper(value: string): WallpaperClassification {
 
 const ALLOWED_IMAGE_PREFIX = "/wallpapers/";
 
+export class UnsafeImagePrefixError extends Error {
+	constructor(prefix: string) {
+		super(`Image wallpaper path must live under ${prefix}`);
+		this.name = "UnsafeImagePrefixError";
+	}
+}
+
 export function resolveImageWallpaperUrl(imagePath: string): string {
 	if (
 		imagePath.startsWith("http://") ||
@@ -48,10 +55,7 @@ export function resolveImageWallpaperUrl(imagePath: string): string {
 	}
 	const withLeadingSlash = imagePath.startsWith("/") ? imagePath : `/${imagePath}`;
 	if (!withLeadingSlash.startsWith(ALLOWED_IMAGE_PREFIX)) {
-		throw new BackgroundLoadError(
-			imagePath,
-			new Error(`Image wallpaper path must live under ${ALLOWED_IMAGE_PREFIX}`),
-		);
+		throw new BackgroundLoadError(imagePath, new UnsafeImagePrefixError(ALLOWED_IMAGE_PREFIX));
 	}
 	try {
 		return getAssetPath(withLeadingSlash.slice(1));