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/utils/assistant-image-source.test.ts

@@ -79,6 +79,19 @@ describe("resolveAssistantImageSource", () => {
     });
   });
 
+  it("normalizes markdown-encoded Windows paths into file RPC requests", () => {
+    expect(
+      resolveAssistantImageSource({
+        source: "C:%5CUsers%5Chanse%5CAppData%5CLocal%5CTemp%5Cpaseo-attachments%5Cimage.png",
+        workspaceRoot: "C:/Users/hanse/eatingkat",
+      }),
+    ).toEqual({
+      kind: "file_rpc",
+      cwd: "C:/",
+      path: "C:/Users/hanse/AppData/Local/Temp/paseo-attachments/image.png",
+    });
+  });
+
   it("falls back to the drive root for Windows absolute paths", () => {
     expect(
       resolveAssistantImageSource({