Update project template assets, layouts, and add TabView template

[niels9001]

Summary

This PR refreshes the default project template assets across all VSIX and dotnet new templates, improves page layouts in the NavigationView template, enhances the MVVM template's sample UI, and introduces a new WinUI TabView App template.

Changes

🎨 Updated template assets

• Replaced all default placeholder images (splash screen, logos, store logo, lock screen) across all 7 template asset locations (C#/C++ SingleProjectPackagedApp, UnitTestApp, NavigationApp, PackagedApp WapProj) with higher-quality versions and a new design that is looking more Fluent (rounded corners)
• Added new Square44x44Logo.targetsize-48_altform-lightunplated.png to all templates — ensures the app icon renders correctly on the taskbar in light theme without showing the accent color plate (used in Windows 8 / 10)

🗂️ NavigationView template improvements

• Updated page layouts that adhere to the right Fluent type ramp and page layout

➕ New TabView App template (winui-tabview)

• New project template featuring a TabView with title bar integration (Mica BaseAlt backdrop, app icon in tab strip header, drag region as title bar).
• Includes AddTab helper, add/close tab handlers, and auto-close on last tab removal.
• Ships with Home and About sample pages using the same Fluent layout pattern.
• Registered in the dotnet new template pack as winui-tabview / winui3-tabview.

🔄 MVVM template improvements

• Replaced the "Update" button with an interactive counter UI: + / - buttons with a bound counter value, demonstrating two-way MVVM data binding with ObservableProperty
  and RelayCommand.
• Removed the Assets folder from MvvmApp (it already pulls assets from SingleProjectPackagedApp during template packaging).