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/utils/explorer-paths.ts

@@ -1,14 +1,10 @@
+import { isAbsolutePath } from "./path";
+
 interface BuildAbsoluteExplorerPathInput {
   workspaceRoot: string;
   entryPath: string;
 }
 
-function isAbsolutePath(pathValue: string): boolean {
-  return (
-    pathValue.startsWith("/") || pathValue.startsWith("\\\\") || /^[A-Za-z]:[\\/]/.test(pathValue)
-  );
-}
-
 export function buildAbsoluteExplorerPath({
   workspaceRoot,
   entryPath,

packages/app/src/utils/inline-path.ts

@@ -1,3 +1,5 @@
+import { isAbsolutePath } from "./path";
+
 export interface InlinePathTarget {
   raw: string;
   path: string;
@@ -183,7 +185,7 @@ export function parseAssistantFileLink(
     };
   }
 
-  if (!trimmed.startsWith("/")) {
+  if (!isAbsolutePath(trimmed)) {
     return null;
   }
 
@@ -195,7 +197,7 @@ export function parseAssistantFileLink(
   }
 
   const normalizedPath = normalizePathToken(decodeURIComponent(parsedUrl.pathname));
-  if (!normalizedPath || !normalizedPath.startsWith("/")) {
+  if (!normalizedPath || !isAbsolutePath(normalizedPath)) {
     return null;
   }
 
@@ -334,7 +336,3 @@ function resolvePathAgainstCwd(pathValue: string, cwd?: string): string | null {
 function normalizePathForCompare(value: string): string {
   return /^[A-Za-z]:/.test(value) ? value.toLowerCase() : value;
 }
-
-function isAbsolutePath(value: string): boolean {
-  return value.startsWith("/") || /^[A-Za-z]:\//.test(value);
-}

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

@@ -0,0 +1,39 @@
+import { describe, expect, it } from "vitest";
+import { isAbsolutePath } from "./path";
+
+describe("isAbsolutePath", () => {
+  it("returns true for Unix absolute paths", () => {
+    expect(isAbsolutePath("/")).toBe(true);
+    expect(isAbsolutePath("/home/user")).toBe(true);
+    expect(isAbsolutePath("/tmp/file.txt")).toBe(true);
+  });
+
+  it("returns true for Windows drive letter paths", () => {
+    expect(isAbsolutePath("C:\\Users")).toBe(true);
+    expect(isAbsolutePath("C:/Users")).toBe(true);
+    expect(isAbsolutePath("d:\\projects")).toBe(true);
+  });
+
+  it("returns true for UNC paths", () => {
+    expect(isAbsolutePath("\\\\server\\share")).toBe(true);
+    expect(isAbsolutePath("\\\\\\\\host\\path")).toBe(true);
+  });
+
+  it("returns false for relative paths", () => {
+    expect(isAbsolutePath("foo/bar")).toBe(false);
+    expect(isAbsolutePath("./relative")).toBe(false);
+    expect(isAbsolutePath("../parent")).toBe(false);
+    expect(isAbsolutePath("")).toBe(false);
+    expect(isAbsolutePath("file.txt")).toBe(false);
+  });
+
+  it("returns false for edge cases that are not absolute paths", () => {
+    expect(isAbsolutePath("")).toBe(false);
+    expect(isAbsolutePath("C:")).toBe(false);
+  });
+
+  it("handles mixed separators in absolute paths", () => {
+    expect(isAbsolutePath("C:/Users\\mixed/path")).toBe(true);
+    expect(isAbsolutePath("/tmp\\mixed/path")).toBe(true);
+  });
+});

packages/app/src/utils/path.ts

@@ -0,0 +1,5 @@
+export function isAbsolutePath(value: string): boolean {
+  return (
+    value.startsWith("/") || value.startsWith("\\\\") || /^[A-Za-z]:[\\/]/.test(value)
+  );
+}