Fix Windows image previews (#1811)

* fix(images): preserve Windows preview paths

Provider image markdown used escaped backslash paths on Windows, which markdown parsing could turn into encoded drive paths that the app treated as workspace-relative. Emit drive-letter paths as file URIs and normalize existing encoded local paths before file preview.

* test(server): accept file URI image sources

Windows provider image output now renders drive-letter paths as file URIs. Decode those markdown sources before filesystem assertions so the image rendering tests keep checking the materialized file.

* fix(images): keep plain local paths literal

Limit legacy URI decoding to markdown-encoded Windows drive paths so ordinary local paths with percent sequences remain unchanged. Document why provider image markdown matching still accepts old doubled-backslash history.

packages/app/src/attachments/utils.ts

@@ -139,11 +139,41 @@ export function pathToFileUri(path: string): string {
   return `file:///${path.replace(/\\/g, "/")}`;
 }
 
+function decodeFilePathSource(source: string): string {
+  try {
+    return decodeURIComponent(source);
+  } catch {
+    return source;
+  }
+}
+
+function normalizeWindowsDrivePath(path: string): string {
+  if (!/^[A-Za-z]:[\\/]/.test(path)) {
+    return path;
+  }
+  return path.replace(/\\/g, "/");
+}
+
+function isMarkdownEncodedWindowsDrivePath(source: string): boolean {
+  return /^[A-Za-z]:(?:%5[Cc]|%2[Ff])/.test(source);
+}
+
 export function fileUriToPath(uri: string): string {
   if (!uri.startsWith("file://")) {
     return uri;
   }
-  return decodeURIComponent(uri.replace(/^file:\/\//, ""));
+  const decodedPath = decodeFilePathSource(uri.replace(/^file:\/\//, ""));
+  return normalizeWindowsDrivePath(decodedPath.replace(/^\/([A-Za-z]:[\\/])/, "$1"));
+}
+
+export function localFileSourceToPath(source: string): string {
+  let path = source;
+  if (source.startsWith("file://")) {
+    path = fileUriToPath(source);
+  } else if (isMarkdownEncodedWindowsDrivePath(source)) {
+    path = decodeFilePathSource(source);
+  }
+  return normalizeWindowsDrivePath(path);
 }
 
 export function getFileExtensionFromName(fileName: string | null | undefined): string {