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/path.ts

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