Implement new card-based home page (#214)

[xenobiasoft]

Summary

• Replaces the accordion-style home page with three navigation cards: Profile (Create/Manage), Start New Game, and Browse Game List
• Adds dedicated Select Difficulty page (/select-difficulty) and Game List page (/games) in both React and Blazor
• Simplifies usePlayerService to be localStorage-only (removes backend profile verification and auto-navigate side effects); exposes isNewPlayer flag
• Silent legacy migration: sudoku-alias → sudoku-profile (with new GUID) runs on home page mount with no visible feedback
• New players see "Create Profile" card enabled (primary styling) and game cards disabled with helper text; returning players see all cards enabled
• Feature parity across Blazor Server and React/Vite frontends

Test plan

• All 154 React tests pass (npm test)
• All 724 .NET tests pass (dotnet test)
• New player flow: visit /, see "Create Profile" highlighted and game cards disabled with helper text
• Returning player flow: all three cards enabled, "Manage Profile" label on profile card
• Legacy player flow: sudoku-alias present → silently migrated to sudoku-profile on page load, rendered as returning player
• Start New Game navigates to /select-difficulty; each difficulty navigates to /new/{difficulty}
• Browse Game List navigates to /games; games load, load/delete actions work, empty state shown when no games
• Direct visit to /select-difficulty or /games without a profile redirects to /
• Back buttons on both new pages return to /

Closes #214

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