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.test.ts

@@ -1,5 +1,12 @@
 import { describe, expect, it } from "vitest";
-import { createImageSourceCacheKey, parseDataUrl, parseImageDataUrl, pathToFileUri } from "./utils";
+import {
+  createImageSourceCacheKey,
+  fileUriToPath,
+  localFileSourceToPath,
+  parseDataUrl,
+  parseImageDataUrl,
+  pathToFileUri,
+} from "./utils";
 
 describe("pathToFileUri", () => {
   it("converts POSIX absolute paths to file URIs", () => {
@@ -23,6 +30,24 @@ describe("pathToFileUri", () => {
   });
 });
 
+describe("fileUriToPath", () => {
+  it("converts Windows drive-letter file URIs back to paths", () => {
+    expect(fileUriToPath("file:///C:/Users/file.txt")).toBe("C:/Users/file.txt");
+  });
+});
+
+describe("localFileSourceToPath", () => {
+  it("decodes markdown-encoded Windows drive-letter paths", () => {
+    expect(localFileSourceToPath("C:%5CUsers%5Cfile.txt")).toBe("C:/Users/file.txt");
+  });
+
+  it("preserves literal percent sequences in plain local paths", () => {
+    expect(localFileSourceToPath("/tmp/image%20with%20literal%20percent.png")).toBe(
+      "/tmp/image%20with%20literal%20percent.png",
+    );
+  });
+});
+
 describe("parseDataUrl", () => {
   it("accepts base64 data URLs with media-type parameters", () => {
     expect(parseDataUrl("data:image/png;charset=utf-8;name=preview;base64,AAECAw==")).toEqual({