Clarify .NET 10 guidance and refactor auth storage keys

[Copilot]

Updated repository documentation to reflect .NET 10 usage and refactored authentication storage to use centralized constants instead of magic strings.

Documentation updates

• Updated AGENTS.md files to reference .NET 10 instead of .NET 9
• Corrected Playwright browser installation path from net9.0 to net10.0

Authentication storage refactoring

• Introduced AuthStorageKeys constants class for centralized storage key management
• Replaced magic strings throughout CustomAuthStateProvider with named constants
• Added user caching to localStorage to reduce redundant API calls during auth state checks
• Implemented stale cache cleanup when cached user doesn't match current userId

// Before
await _localStorage.SetItemAsync("currentUser", user);

// After  
await _localStorage.SetItemAsync(AuthStorageKeys.CurrentUserStorageKey, user);

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