fix: replace Unix path assumptions with cross-platform isAbsolutePath

Windows daemon paths like C:\Users\... don't start with "/", breaking
the explorer sidebar, checkout status, terminals, and file attachments
when connected to a Windows daemon. Consolidate three private
isAbsolutePath implementations into a shared utility and use it
everywhere, with correct file URI conversion for Windows and UNC paths.

packages/app/src/attachments/native/native-file-attachment-store.ts

@@ -1,4 +1,5 @@
 import { createLocalFileAttachmentStore } from "@/attachments/local-file-attachment-store";
+import { isAbsolutePath } from "@/utils/path";
 
 export function createNativeFileAttachmentStore() {
   return createLocalFileAttachmentStore({
@@ -8,8 +9,17 @@ export function createNativeFileAttachmentStore() {
       if (attachment.storageKey.startsWith("file://")) {
         return attachment.storageKey;
       }
-      if (attachment.storageKey.startsWith("/")) {
-        return `file://${attachment.storageKey}`;
+      if (isAbsolutePath(attachment.storageKey)) {
+        if (attachment.storageKey.startsWith("/")) {
+          return `file://${attachment.storageKey}`;
+        }
+
+        // UNC paths: \\server\share -> file://server/share
+        if (attachment.storageKey.startsWith("\\\\")) {
+          return `file:${attachment.storageKey.replace(/\\/g, "/")}`;
+        }
+
+        return `file:///${attachment.storageKey.replace(/\\/g, "/")}`;
       }
       return attachment.storageKey;
     },

packages/app/src/attachments/utils.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+import { pathToFileUri } from "./utils";
+
+describe("pathToFileUri", () => {
+  it("converts POSIX absolute paths to file URIs", () => {
+    expect(pathToFileUri("/home/user/file.txt")).toBe("file:///home/user/file.txt");
+  });
+
+  it("converts Windows drive-letter paths to file URIs", () => {
+    expect(pathToFileUri("C:\\Users\\file.txt")).toBe("file:///C:/Users/file.txt");
+  });
+
+  it("converts UNC paths to host-based file URIs", () => {
+    expect(pathToFileUri("\\\\server\\share\\dir")).toBe("file://server/share/dir");
+  });
+
+  it("passes through file URIs unchanged", () => {
+    expect(pathToFileUri("file:///already/uri")).toBe("file:///already/uri");
+  });
+
+  it("passes through relative paths unchanged", () => {
+    expect(pathToFileUri("relative/path")).toBe("relative/path");
+  });
+});

packages/app/src/attachments/utils.ts

@@ -1,4 +1,5 @@
 import { generateMessageId } from "@/types/stream";
+import { isAbsolutePath } from "@/utils/path";
 
 export function generateAttachmentId(): string {
   return `att_${generateMessageId()}`;
@@ -53,10 +54,21 @@ export function pathToFileUri(path: string): string {
   if (path.startsWith("file://")) {
     return path;
   }
+
+  if (!isAbsolutePath(path)) {
+    return path;
+  }
+
   if (path.startsWith("/")) {
     return `file://${path}`;
   }
-  return path;
+
+  // UNC paths: \\server\share -> file://server/share
+  if (path.startsWith("\\\\")) {
+    return `file:${path.replace(/\\/g, "/")}`;
+  }
+
+  return `file:///${path.replace(/\\/g, "/")}`;
 }
 
 export function fileUriToPath(uri: string): string {