refactor(samples): consolidate shared components across Blazor samples

[mckaragoz]

Summary

• Introduce Sample.Shared and Sample.Shared.Hub projects to reduce code duplication.
• Unify pages, layouts, dialogs, and authentication examples across Blazor Server, Standalone WASM, and Interactive WASM.
• Preserve hosting-specific authentication flows and configurations.
• Simplify sample projects and update documentation.

No intended changes to UltimateAuth's core authentication behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.98%. Comparing base (9632eaa) to head (41a052e).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #81   +/-   ##
=======================================
  Coverage   85.98%   85.98%           
=======================================
  Files         660      660           
  Lines       18275    18275           
  Branches     2249     2249           
=======================================
  Hits        15714    15714           
  Misses       1810     1810           
  Partials      751      751           

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