iOS: SwiftUI multiplatform section

[mstrlc]

Summary

Adds a new SwiftUI Multiplatform page to the iOS section of the Engineering Handbook. Covers platform specifics, Xcode setup, navigation paradigms (TabView vs NavigationSplitView), adaptive components, and Liquid Glass — intended as both internal reference and client upsell material.

Changes

• New page ios_multiplatform.md with sections on platform specifics, Xcode setup, platform-specific code patterns, navigation paradigms, detail presentation, search, adaptive Table, Liquid Glass, and when to use which
• 21 screenshots illustrating each navigation pattern across iPhone, iPad, and Mac
• index.md and mkdocs.yml updated to include the new page

Notes

• TabView (.sidebarAdaptable) patterns require iOS 18 / macOS 15+; Liquid Glass requires iOS 26 / macOS 26+
• FuturedKit macOS compatibility gaps are documented (SceneDelegate, image pickers, TextStyle/Font system)

[eRDe33]

Za me super, napadla me tam jenom ta jedna drobnost. 👍

[eRDe33]

Mozna bych tu explicitne zminil, ze je to mozne i s jednim multiplatformnim targetem. Prvni co me totiz napadlo bylo "Ha, ale platform-specific files se da udelat jenom pomoci pridani do ruznych targetu.", ale neni to pravda, i kdyz mas pouze jeden multiplatformni target, i tak muzes ve File Inspectoru nastavit spravne Target Membership pouze konkretni platformne.

Suggested change

	Keep `#if` blocks small and push them to the edges of your view hierarchy. Wrapping an entire screen in `#if` is a sign the screen should be split into platform-specific files instead.
	Keep #if blocks small and push them to the edges of your view hierarchy. Wrapping an entire screen in #if is a sign the screen should be split into platform-specific files instead.
	Even with a single multiplatform target, you can control which platforms a file compiles for via Target Membership in the File Inspector — no need for a separate target just to achieve per-file platform separation.