fix: showSaveFilePicker from external websites

Saving a file via puter.ui.showSaveFilePicker from a third-party website
(popup flow) consistently failed with a DataCloneError alert after
clicking Save, and saving over an existing filename showed a raw error
instead of the Replace/Cancel prompt.

Two bugs:

1. privacy_aware_path is a curried factory (world => fspath => ...), and
   initgui.js is the only module that imports it directly — so the popup
   save handler's privacy_aware_path(res.path) returned the inner
   function, which postMessage cannot structured-clone. Every other call
   site resolves the bare name to the correctly bound
   window.privacy_aware_path global, which is why only the external-site
   popup flow was broken. Use the global at the call site and import the
   factory under a distinct name so a bare call can't silently resolve
   to it again.

2. The v2 backend returns `conflict` for a same-name write, but the v1
   wire contract is `item_with_same_name_exists` + `entry_name`, which
   the GUI's save dialogs key on to offer the overwrite prompt. Restore
   the legacy code/field on the write-conflict error and carry
   HttpError.fields through the /batch per-op error serializer.

Verified end-to-end against a local backend: fresh save resolves the
caller's promise with the signed saved_file and closes the popup;
saving an existing name shows Replace/Cancel and Replace overwrites.
Backend suite shows no new failures.

src/backend/controllers/fs/LegacyFSController.ts

@@ -167,8 +167,7 @@ export class LegacyFSController extends PuterController {
 
         router.get('/get-launch-apps', apiOptions, async (req, res) => {
             const recommendedSvc = this.services.recommendedApps as unknown as
-                | { getRecommendedApps?: () => Promise<unknown[]> }
-                | undefined;
+                { getRecommendedApps?: () => Promise<unknown[]> } | undefined;
             const recommended = recommendedSvc?.getRecommendedApps
                 ? await recommendedSvc.getRecommendedApps()
                 : [];
@@ -640,9 +639,7 @@ export class LegacyFSController extends PuterController {
             // Trash, and `null`/`{}` when restoring. See
             // `src/gui/src/helpers.js` → `window.move_items`.
             newMetadata: (body.new_metadata ?? undefined) as
-                | Record<string, unknown>
-                | null
-                | undefined,
+                Record<string, unknown> | null | undefined,
         });
         const oldPath = source.path;
         await this.#emitGuiEvent('outer.gui.item.moved', moved, {
@@ -1071,8 +1068,7 @@ export class LegacyFSController extends PuterController {
         }
 
         type SignedOrEmpty =
-            | (SignedFile & { path?: string })
-            | Record<string, never>;
+            (SignedFile & { path?: string }) | Record<string, never>;
         const result: { signatures: SignedOrEmpty[]; token?: string } = {
             signatures: [],
         };
@@ -1612,10 +1608,7 @@ export class LegacyFSController extends PuterController {
         const subjectRef = body.subject;
         const appRef = body.app;
         const mode = (getString(body, 'mode') ?? 'read') as
-            | 'see'
-            | 'list'
-            | 'read'
-            | 'write';
+            'see' | 'list' | 'read' | 'write';
         if (!subjectRef || !appRef)
             throw new HttpError(400, '`subject` and `app` are required', {
                 legacyCode: 'bad_request',
@@ -2271,12 +2264,21 @@ export class LegacyFSController extends PuterController {
 
     #serializeBatchError(err: unknown): Record<string, unknown> {
         if (err instanceof HttpError) {
-            return {
+            const payload: Record<string, unknown> = {
                 error: true,
                 status: err.statusCode,
                 message: err.message,
                 code: err.legacyCode ?? err.code,
             };
+            // Same as the terminal errorHandler: extra fields (e.g.
+            // `entry_name` on item_with_same_name_exists) ride along
+            // without clobbering the canonical slots.
+            if (err.fields) {
+                for (const [k, v] of Object.entries(err.fields)) {
+                    if (!(k in payload)) payload[k] = v;
+                }
+            }
+            return payload;
         }
         if (err instanceof Error) {
             return { error: true, status: 500, message: err.message };

src/backend/services/fs/FSService.ts

@@ -558,10 +558,20 @@ export class FSService extends PuterService {
                 );
             }
             if (existingEntry && !normalizedInput.overwrite) {
+                // v1 wire contract: clients (the GUI's save dialogs among
+                // them) key on `item_with_same_name_exists` + `entry_name`
+                // to offer an overwrite prompt.
                 throw new HttpError(
                     409,
                     'A file already exists at this path and overwrite was not requested',
-                    { legacyCode: 'conflict' },
+                    {
+                        legacyCode: 'item_with_same_name_exists',
+                        fields: {
+                            entry_name: pathPosix.basename(
+                                normalizedInput.path,
+                            ),
+                        },
+                    },
                 );
             }
 

src/gui/src/initgui.js

@@ -54,7 +54,10 @@ import { LaunchOnInitService } from './services/LaunchOnInitService.js';
 import { LocaleService } from './services/LocaleService.js';
 import { ProcessService } from './services/ProcessService.js';
 import { ThemeService } from './services/ThemeService.js';
-import { privacy_aware_path } from './util/desktop.js';
+// Curried: takes `{ window }` and returns the path mapper. Import under a
+// factory name so a bare `privacy_aware_path(path)` call in this module can't
+// silently resolve to the factory — use `window.privacy_aware_path` instead.
+import { privacy_aware_path as privacy_aware_path_factory } from './util/desktop.js';
 
 const postAuthActions = async (action) => {
     // -------------------------------------------------------------------------------------
@@ -378,7 +381,7 @@ const postAuthActions = async (action) => {
                                         metadataURL: file_signature.metadata_url,
                                         type: file_signature.type,
                                         uid: file_signature.uid,
-                                        path: privacy_aware_path(res.path),
+                                        path: window.privacy_aware_path(res.path),
                                     },
                                 }, '*');
 
@@ -2076,7 +2079,7 @@ $(document).on('contextmenu', '.disable-context-menu', function (e) {
 });
 
 // util/desktop.js
-window.privacy_aware_path = privacy_aware_path({ window });
+window.privacy_aware_path = privacy_aware_path_factory({ window });
 
 $(window).on('system-logout-event', function () {
     // Clear cookie