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,