fix(stack-files): confirm before overwriting an existing upload target (#1204)

* fix(stack-files): confirm before overwriting an existing upload target

Same-name uploads previously truncated the existing file silently. A
user dragging a file with a name that matched an in-place file
destroyed the original with no warning and no undo.

The upload route now reads ?overwrite=0|1. When the flag is not set
and the target already exists, the server returns 409 FILE_EXISTS
and the original file is untouched. The frontend opens a confirm
dialog and retries with overwrite=1 on the user's approval; cancel
keeps the original.

A new pathExists helper on FileSystemService performs the existence
check through the same path-resolution barrier as the write so a
malicious relPath cannot bypass the conflict check. UploadConflictError
is exported so callers can distinguish the conflict case from generic
upload failures without parsing error strings.

* fix(stack-files): distinct DIR_EXISTS code, drop INVALID_PATH swallow in existence check

frontend/src/components/files/FileUploadDropzone.tsx

@@ -1,7 +1,8 @@
-import { useRef } from 'react';
+import { useRef, useState } from 'react';
 import { UploadCloud } from 'lucide-react';
+import { ConfirmModal } from '@/components/ui/modal';
 import { toast } from '@/components/ui/toast-store';
-import { uploadStackFile } from '@/lib/stackFilesApi';
+import { uploadStackFile, UploadConflictError } from '@/lib/stackFilesApi';
 
 const MAX_BYTES = 25 * 1024 * 1024; // 25 MB
 
@@ -19,29 +20,38 @@ export function FileUploadDropzone({
   onUploaded,
 }: FileUploadDropzoneProps) {
   const inputRef = useRef<HTMLInputElement>(null);
+  const [conflict, setConflict] = useState<File | null>(null);
 
   if (!canEdit) return null;
 
-  const handleFile = async (file: File) => {
-    if (file.size > MAX_BYTES) {
-      toast.error('File exceeds 25 MB.');
-      return;
-    }
+  const runUpload = async (file: File, overwrite: boolean): Promise<void> => {
     const loadingId = toast.loading(`Uploading ${file.name}...`);
     try {
-      await uploadStackFile(stackName, currentDir, file);
-      toast.success('Uploaded.');
+      await uploadStackFile(stackName, currentDir, file, { overwrite });
+      toast.success(overwrite ? 'Replaced.' : 'Uploaded.');
       onUploaded();
     } catch (e: unknown) {
+      if (e instanceof UploadConflictError) {
+        setConflict(file);
+        return;
+      }
       toast.error(e instanceof Error ? e.message : 'Upload failed.');
     } finally {
       toast.dismiss(loadingId);
     }
   };
 
+  const handleFile = (file: File) => {
+    if (file.size > MAX_BYTES) {
+      toast.error('File exceeds 25 MB.');
+      return;
+    }
+    void runUpload(file, false);
+  };
+
   const handleChange = (ev: React.ChangeEvent<HTMLInputElement>) => {
     const file = ev.target.files?.[0];
-    if (file) void handleFile(file);
+    if (file) handleFile(file);
     ev.target.value = '';
   };
 
@@ -69,6 +79,25 @@ export function FileUploadDropzone({
         <UploadCloud className="w-3.5 h-3.5 shrink-0" strokeWidth={1.5} />
         Upload file
       </div>
+
+      <ConfirmModal
+        open={conflict !== null}
+        onOpenChange={(next) => { if (!next) setConflict(null); }}
+        onCancel={() => setConflict(null)}
+        kicker="FILES · REPLACE EXISTING"
+        title="Replace existing file?"
+        description={conflict ? `${conflict.name} already exists in this folder.` : ''}
+        confirmLabel="Replace"
+        onConfirm={() => {
+          const file = conflict;
+          setConflict(null);
+          if (file) void runUpload(file, true);
+        }}
+      >
+        <p className="text-sm text-muted-foreground">
+          {conflict?.name ?? 'The file'} already exists. Replacing it overwrites the current contents and cannot be undone.
+        </p>
+      </ConfirmModal>
     </>
   );
 }