feat: bump TodoApp.Avalonia sample from Avalonia 11 to 12 (12.1.0)

[adrianhall]

Summary

Bumps the TodoApp.Avalonia sample from Avalonia 11 to 12.1.0 (the latest release at time of implementation) for general currency, since 11.x will presumably reach end-of-support once 12 is established. This is a follow-up to #499 (which fixed NU1903 via an 11.3.18 patch bump only) and is its own scoped change per #507.

Closes #507

Changes

All changes verified against the official Avalonia 12 breaking changes doc:

• Package versions: bump the shared AvaloniaVersion property (Directory.Build.props) from 11.3.18 to 12.1.0, used by all Avalonia.* PackageReferences across the shared, Desktop, Android, and iOS projects.
• Avalonia.Diagnostics removed in v12: drop the Debug-only PackageReference from TodoApp.Avalonia.csproj and TodoApp.Avalonia.Desktop.csproj (no AttachDevTools() call exists in the sample, so this is a clean removal).
• Binding plugins removed in v12: remove BindingPlugins.DataValidators.RemoveAt(0); and its Avalonia.Data.Core* usings from App.axaml.cs. Behavior is preserved because v12 disables the data-annotations validation plugin by default.
• Android now implements IActivityApplicationLifetime instead of ISingleViewApplicationLifetime: add a MainViewFactory-based branch in App.axaml.cs, ordered before the existing ISingleViewApplicationLifetime check (which is still required for iOS/browser/embedded Linux, per Avalonia's own migration guidance).
• Android app-initialization rework:
  • Add TodoApp.Avalonia.Android/Application.cs: a new [Application]-attributed Application : AvaloniaAndroidApplication<App> class hosting the CustomizeAppBuilder override (.WithInterFont()).
  • Change MainActivity from generic AvaloniaMainActivity<App> to non-generic AvaloniaMainActivity (the CreateAppBuilder/CustomizeAppBuilder virtual methods were removed from the generic base class in v12; that logic now lives in Application.cs).
  • Wire up android:name=".Application" on the <application> element in AndroidManifest.xml.
• MainView.axaml: rename Watermark="Add a new Item" → PlaceholderText="Add a new Item" (old property is kept but obsolete in v12).
• Trim-analyzer comment cleanup: drop the now-stale "Avalonia's BindingPlugins.DataValidators" mention from the SuppressTrimAnalysisWarnings justification comment in the Android/iOS .csproj files, since that code path no longer exists — only EF Core's DbContext(DbContextOptions) ctor remains a relevant trim-analyzer noise source.
• iOS AppDelegate.cs: no changes required — confirmed against the breaking-changes doc that AvaloniaAppDelegate.CustomizeAppBuilder was not removed for iOS (only Window becoming null-until-scene-attached, which this sample doesn't touch), and confirmed via a real CI build that it still compiles unchanged.

Confirmed NOT applicable (verified via source grep against the breaking-changes doc)

Gestures.* attached events, clipboard/IDataObject APIs, GotFocusEventArgs, WindowState-in-styles, TextBlock.LetterSpacing, SystemDecorations/ExtendClientAreaChromeHints, KeyboardNavigationHandler, custom Screen construction, Direct2D1/Blazor/Tizen backends, headless xUnit/NUnit tests — no matches found in the sample source.

Related

• Investigate resolving NU1903 for Tmds.DBus.Protocol in Avalonia desktop sample #499 — Avalonia-side NU1903 fix (11.3.x patch bump), already resolved without this major bump.
• TodoApp.Avalonia.Android: SkiaSharp native library not 16 KB page-size aligned (XA0141) #522 — TodoApp.Avalonia.Android XA0141 (libSkiaSharp.so not 16 KB page-size aligned), closed as won't-fix pending this migration. Verified as part of this PR: the CI Android build log for this change shows 0 Warning(s) — the XA0141 warning is gone now that Avalonia 12.1.0 pulls in SkiaSharp 3.119.4 (up from the 2.x line pinned by Avalonia 11.3.18).

Testing

• dotnet build (Debug and Release) succeeds locally for the shared TodoApp.Avalonia and TodoApp.Avalonia.Desktop projects.
• The Android and iOS heads require their respective mobile workloads (not installed locally); validated via the Build Samples GitHub Actions workflow instead:
  • todoapp-avalonia / desktop ✅
  • todoapp-avalonia / android ✅ (0 warnings — confirms the TodoApp.Avalonia.Android: SkiaSharp native library not 16 KB page-size aligned (XA0141) #522 XA0141 fix)
  • todoapp-avalonia / ios ✅
  • all-samples-built ✅
  • Run: https://github.com/adrianhall/CommunityToolkit-Datasync/actions/runs/29151193060
• No tests/ project references TodoApp.Avalonia, so dotnet test is unaffected by this change (build-only verification, matching CI's own scope for this sample).