feat: fallback to UIImage(named:) for custom symbol/image sets in tab…

[HaywhyD]

Summary

Adds UIImage(named:) fallback to CupertinoTabBarPlatformView so that CNTabBar can render custom SF Symbols from the app's Xcode asset catalog, in addition to Apple's built-in SF Symbols.

What changed

All 3 buildItems functions in CupertinoTabBarPlatformView.swift now try:

image = UIImage(systemName: symbols[i])   // built-in SF Symbols
    ?? UIImage(named: symbols[i])          // custom symbols from Assets.xcassets

Non-breaking — existing code using built-in SF Symbol names works exactly as before.

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Usage Guide

1. Create Custom SF Symbols

Important: Custom symbols must be created using Apple's SF Symbols app — regular SVGs will not work.

1. Open the SF Symbols app (free download from Apple)
2. Create or import your custom symbol
3. Export: Right-click → Export Symbol → save as .svg

2. Add to Xcode Asset Catalog

1. Open your Flutter project's iOS workspace: open ios/Runner.xcworkspace
2. Navigate to Runner → Assets.xcassets
3. Drag your exported .svg into the asset catalog — Xcode creates a Symbol Image Set (.symbolset)
4. Note the asset name (filename without extension, e.g. my-custom-icon)

3. Use in Dart

CNTabBarItem(
  label: 'Home',
  icon: CNSymbol('my-custom-icon', size: 18),
)

4. Naming

⚠️ Custom symbol names must NOT conflict with built-in SF Symbol names (systemName is tried first).

Use a unique prefix: myapp-home ✅, house ❌ (conflicts with built-in SF Symbol)

5. Android

⚠️ On Android, CNTabBar falls back to CupertinoTabBar which does NOT render custom icons (shows circles). Use a platform check and provide a separate Android nav bar.