Replace iOS18 with iOS26 across codebase

[jamespepper81]

This pull request updates the app's support for the iOS "liquid glass" UI effect, raising the minimum iOS version required for these advanced blur materials from iOS 18 to iOS 26. All relevant code, documentation, and helper functions have been updated to reflect this new baseline. The changes ensure that the liquid glass effect and related UI features are only enabled on iOS 26 or later, with appropriate fallbacks for earlier versions.

Platform version update for liquid glass effect:

• All references to iOS 18+ for the liquid glass UI effect have been changed to iOS 26+ throughout the codebase and documentation, including component descriptions, comments, and user-facing docs. [1] [2] [3] [4] [5] [6] [7] [8] [9]

Code and helper function changes:

• The helper function isIOS18OrHigher() has been renamed and updated to isIOS26OrHigher(), with logic and usage updated accordingly in both utils/platform.ts and all call sites. [1] [2] [3]
• All blur tint helper functions (getLiquidGlassTint, getUltraThinMaterialTint, getThinMaterialTint) now check for iOS 26+ instead of iOS 18+. [1] [2] [3]

Documentation updates:

• The implementation details, user experience notes, and testing instructions in both main and archive documentation have been revised to reference iOS 26 instead of iOS 18, ensuring clarity for future developers and users. [1] [2] [3] [4] [5] [6] [7]