feat(apple): generate type-safe L10n accessors and migrate all key literals

Replaces every stringly-typed localization key in the Apple app with
compile-time-checked accessors generated from localization/strings.json.
A mistyped key is now a build error instead of a silent fallback to the
raw key at runtime. The runtime path is unchanged: plain keys are
String.LocalizationValue constants resolved with String(localized:) and
Apple's String Catalog still does the lookup; keys with arguments become
typed, named functions applying args through String(format:).

Generator: new renderSwiftAccessors emits apple/VniDrop/Generated/L10n.swift,
wired into generate. Renamed generic arg1/arg2 tokens on four keys to
semantic names (receiver, transferName, deviceId) and updated their context
notes; positional output is unchanged so .xcstrings (bar the 4 comments)
and the Android XML regenerate identical.

Migration: every key-carrying value flipped to String.LocalizationValue
end to end, resolved only at the leaf. Zero key literals and zero
LocalizedStringKey remain in app or test code. macOS build passes; iOS
test run pending.

apple/VniDrop/UI/Navigation/AppDestination.swift

@@ -8,11 +8,11 @@ enum AppDestination: String, CaseIterable, Identifiable {
 
 	var id: String { rawValue }
 
-	var labelKey: String {
+	var labelKey: String.LocalizationValue {
 		switch self {
-		case .send: return "nav_send"
-		case .receive: return "nav_receive"
-		case .settings: return "nav_settings"
+		case .send: return L10n.Nav.send
+		case .receive: return L10n.Nav.receive
+		case .settings: return L10n.Nav.settings
 		}
 	}