feat: Add Simple theme default and opt-in sample content for recommended preset

[Kunal22shah]

GitHub Issue (If applicable): closes #2019

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Pull request overview

This PR updates the unoapp template’s Recommended preset to default to the Simple theme and generate a blank single-page experience by default, while making the previous multi-page/sample flow opt-in via a new sample parameter.

Changes:

• Add a sample template parameter (useSampleContent) and gate the sample UI, navigation routes, and viewmodel logic behind it.
• Switch the Recommended preset default theme to Simple and wire Simple theme resources into XAML + C# markup variants.
• Update UI tests and add a detailed spec document describing the intended preset behavior.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/Presentation/MainViewModel.cs	Gate Second-page sample navigation/state behind useSampleContent.
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/Presentation/MainModel.cs	Gate MVUX sample navigation/state behind useSampleContent.
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/Presentation/MainPage.xaml	Render sample page UI only when useSampleContent; otherwise show “Hello Uno Platform!”.
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/Presentation/MainPage.xaml.cs	Same as XAML variant for C# Markup: sample UI vs blank page.
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/MyExtensionsApp.1.csproj	Add SimpleTheme UnoFeature when useSimple is selected.
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/App.xaml	Add Simple theme resource dictionaries (toolkit + non-toolkit).
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/App.recommended.cs	Add Simple theme loading for C# markup; gate SecondPage route registration on useSampleContent.
src/Uno.Templates/content/unoapp/MyExtensionsApp.1/App.blank.cs	Add Simple theme loading for C# markup in blank app variant.
src/Uno.Templates/content/unoapp/MyExtensionsApp.1.UITests/Given_MainPage.cs	Update smoke test to only navigate to SecondPage when sample content is enabled.
src/Uno.Templates/content/unoapp/.template.config/template.json	Add sample parameter, useSampleContent computed value, Recommended default theme=Simple, and sample exclusions.
src/Uno.Templates/content/unoapp/.template.config/TemplateWizard.json	Add Simple theme choice + new sample feature option; set Recommended defaults accordingly.
specs/recommended-preset-blank-page.md	New spec documenting the intended simplified Recommended preset output.

Comments suppressed due to low confidence (1)

src/Uno.Templates/content/unoapp/.template.config/TemplateWizard.json:146

• The Recommended preset now sets appTheme to "simple", but the preset still enables "dspGenerator": "true". In this template, DSP import is Material-specific (and the wizard metadata requires appTheme == material), so this combination is inconsistent and the DSP option will be ignored/invalid for the default Recommended configuration. Consider setting dspGenerator to false for the Recommended preset when appTheme is simple (or updating the DSP constraints/implementation if it is intended to support Simple).

				"Parameters": {
					"appTheme": "simple",
					"architecture": "mvux",
					"authentication": "none",
					"configuration": "true",
					"cultures": "es|fr|pt-BR",
					"dependencyInjection": "true",
					"http": "kiota",
					"localization": "true",
					"logging": "default",
					"markup": "xaml",
					"mediaPlayerElement": "false",
					"navigation": "regions",
					"platforms": "android|ios|wasm|desktop",
					"rider": "true",
					"sample": "false",
					"server": "false",
					"skipRestore": "true",
					"tests": "none",
					"tfm": "net10.0",
					"toolkit": "true",
					"vscode": "true",

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