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;
     },