Migrate GUI to Avalonia 12

[vaceslav]

Problem

Dependabot opened three PRs (#100, #101, #102) bumping Avalonia packages from 11.3.10 to 12.0.3. All fail CI on every platform with:

TriasDev.Templify.Gui/App.axaml.cs(81,13): error CS0122:
  'BindingPlugins' is inaccessible due to its protection level
TriasDev.Templify.Gui/App.axaml.cs(86,13): error CS0122:
  'BindingPlugins' is inaccessible due to its protection level

Two issues:

1. Breaking API change — In Avalonia 12, BindingPlugins is no longer publicly configurable. The DataAnnotationsValidationPlugin is now disabled by default, so the manual removal in DisableAvaloniaDataAnnotationValidation() is obsolete and must be deleted.
2. Split bumps — Dependabot created three PRs each bumping only a subset of Avalonia packages (Desktop / Fonts.Inter / Themes.Fluent), leaving the others on 11.3.10. Avalonia packages must move together; none of these PRs is mergeable individually.

Plan

• Remove DisableAvaloniaDataAnnotationValidation() and unused Avalonia.Data.Core / Avalonia.Data.Core.Plugins / System.Linq usings in App.axaml.cs.
• Bump all Avalonia packages (Avalonia, Avalonia.Desktop, Avalonia.Themes.Fluent, Avalonia.Fonts.Inter, Avalonia.Diagnostics) to 12.0.3 in TriasDev.Templify.Gui.csproj.
• Verify build and tests pass on all matrix targets.
• After merge, close chore: Bump Avalonia and Avalonia.Desktop #100, chore: Bump Avalonia and Avalonia.Fonts.Inter #101, chore: Bump Avalonia and Avalonia.Themes.Fluent #102.
• Follow-up: group Avalonia* packages in dependabot.yml so future bumps land in a single PR.

References

• Avalonia 12 breaking changes
• Discussion #20717 — BindingPlugins inaccessible