Avoid over-narrowing browser path checks

The browser path helper briefly acted as a type predicate, which made existing non-browser path branches narrow to never under the production build. The callers now guard nullable paths explicitly while the helper stays a plain boolean check.

Constraint: ProjectSetupModal depends on checking both browserfs paths and regular filesystem strings in the same helper flow.
Confidence: high
Scope-risk: narrow
Tested: npm test -- --run src/components/__tests__/GenerateImageNode.test.tsx src/store/execution/__tests__/nanoBananaExecutor.test.ts src/components/__tests__/GenerationComposer.test.tsx
Tested: npx tsc --noEmit --pretty false filtered to touched browser path files
Not-tested: Full tsc remains blocked by existing test typing errors outside this change scope

src/components/nodes/GenerateImageNode.tsx

@@ -341,7 +341,7 @@ export function GenerateImageNode({ id, data, selected }: NodeProps<NanoBananaNo
 
   const loadImageById = useCallback(async (imageId: string) => {
     const effectiveGenerationsPath = generationsPath
-      ?? (isBrowserFileSystemPath(saveDirectoryPath)
+      ?? (saveDirectoryPath && isBrowserFileSystemPath(saveDirectoryPath)
         ? joinBrowserFileSystemPath(saveDirectoryPath, "generations")
         : null);
 

src/store/execution/nanoBananaExecutor.ts

@@ -55,7 +55,7 @@ export async function executeNanoBanana(
 
   const { images: connectedImages, text: connectedText, dynamicInputs } = getConnectedInputs(node.id);
   const effectiveGenerationsPath = generationsPath
-    ?? (isBrowserFileSystemPath(saveDirectoryPath)
+    ?? (saveDirectoryPath && isBrowserFileSystemPath(saveDirectoryPath)
       ? joinBrowserFileSystemPath(saveDirectoryPath, "generations")
       : null);
 

src/store/workflowStore.ts

@@ -2256,7 +2256,7 @@ const workflowStoreImpl: StateCreator<WorkflowStore> = (set, get) => ({
       workflowName: workflow.name,
       saveDirectoryPath: directoryPath || null,
       generationsPath: savedConfig?.generationsPath
-        || (directoryPath && isBrowserFileSystemPath(directoryPath)
+        || (typeof directoryPath === "string" && isBrowserFileSystemPath(directoryPath)
           ? joinBrowserFileSystemPath(directoryPath, "generations")
           : null),
       lastSavedAt: savedConfig?.lastSavedAt || null,

src/utils/browserFileSystem.ts

@@ -40,7 +40,7 @@ declare global {
 
 const memoryHandles = new Map<string, FileSystemDirectoryHandle>();
 
-export function isBrowserFileSystemPath(path: string | null | undefined): path is string {
+export function isBrowserFileSystemPath(path: string | null | undefined): boolean {
   return typeof path === "string" && path.startsWith(PREFIX);
 }