Replace RxSwift with Swift Observation framework

[igorkulman]

Summary

• Remove RxSwift dependency and replace with Swift's native Observation framework
• Support both iOS 18 and iOS 26 using viewWillLayoutSubviews() for observable property tracking
• Simplify reactive patterns across ViewModels and ViewControllers

Changes

Observation Framework Migration

• ViewModels now use @Observable macro instead of RxSwift subjects/observables
• ViewControllers observe property changes via viewWillLayoutSubviews() with
  UIObservationTrackingEnabled in Info.plist
• Removed RxSwift extensions (Reactive+Extensions.swift, RxSwift+WKWebView.swift,
  RxBlocking.swift)

Code Cleanup

• Removed custom cells (FeedCell, RssSourceCell, AboutCell, LibraryCell) in favor of
  UIListContentConfiguration
• Removed UIKit+Preview.swift helper
• Updated DetailViewController to use KVO instead of RxSwift for WKWebView observation
• Simplified async data loading with Swift concurrency

Testing

• Added FeedViewModelTests and RssSourceViewModelTests
• Updated existing tests to work without RxBlocking