Replace the legacy AppIcon.appiconset with an Icon Composer AppIcon.icon
bundle in the target's resources. ASSETCATALOG_COMPILER_APPICON_NAME already
points at "AppIcon"; the .icon back-deploys to the iOS 18.2 target.
The macOS sidebar selection and item icons rendered in the system default
blue because the app had no global accent color — SwiftUI's `.tint` doesn't
reach the AppKit-backed sidebar. Add an AccentColor asset (the exact sRGB of
VniDropColors.brandPurple) and wire ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME
so the accent applies at the OS level everywhere, including the sidebar.
Add a native SwiftUI VniDrop app (Send/Receive/Settings) talking to the
Rust core via generated UniFFI Swift bindings, plus the uniffi-bindgen
helper crate. iOS uses a TabView, macOS a NavigationSplitView sidebar.
