Add ambient type declarations for the potrace module

src/types/potrace.d.ts

@@ -0,0 +1,8 @@
+// Minimal types for the `potrace` package (no official @types).
+declare module "potrace" {
+  type TraceCallback = (err: Error | null, svg: string) => void;
+  export function trace(file: Buffer | string, options: Record<string, unknown>, cb: TraceCallback): void;
+  export function trace(file: Buffer | string, cb: TraceCallback): void;
+  export function posterize(file: Buffer | string, options: Record<string, unknown>, cb: TraceCallback): void;
+  export function posterize(file: Buffer | string, cb: TraceCallback): void;
+}