fix: remove ApplicationIcon from csproj (Android library), fix nullable warning in PhysicalFileStorage

- ApplicationIcon is for Windows desktop executables, not Android libraries
  (it caused CI build failure: 'Could not find file GeneralUpdate.ico')
- PhysicalFileStorage.ReadAllTextAsync: rewrite ternary to async/await
  to resolve CS8619 nullable inference ambiguity

Co-Authored-By: Claude <noreply@anthropic.com>

Author: JusterZhu

src/GeneralUpdate.Avalonia.Android/GeneralUpdate.Avalonia.Android.csproj

@@ -1,7 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>net10.0-android</TargetFramework>
-    <ApplicationIcon>GeneralUpdate.ico</ApplicationIcon>
     <PackageIcon>GeneralUpdate.png</PackageIcon>
     <SupportedOSPlatformVersion>26.0</SupportedOSPlatformVersion>
     <Nullable>enable</Nullable>

src/GeneralUpdate.Avalonia.Android/Services/PhysicalFileStorage.cs

@@ -35,11 +35,11 @@ public Stream OpenWrite(string filePath, bool append)
         return new FileStream(filePath, mode, FileAccess.Write, FileShare.None);
     }
 
-    public Task<string?> ReadAllTextAsync(string filePath, CancellationToken cancellationToken = default)
+    public async Task<string?> ReadAllTextAsync(string filePath, CancellationToken cancellationToken = default)
     {
-        return File.Exists(filePath)
-            ? File.ReadAllTextAsync(filePath, cancellationToken)
-            : Task.FromResult<string?>(null);
+        if (!File.Exists(filePath))
+            return null;
+        return await File.ReadAllTextAsync(filePath, cancellationToken).ConfigureAwait(false);
     }
 
     public Task WriteAllTextAsync(string filePath, string content, CancellationToken cancellationToken = default)