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/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);
-}