refactor(apple): make InvitationError typed and localize NFC prompts

Replace the free-form InvitationError.message(String) case with semantic
cases mapped to L10n keys at the UI boundary (Error.uiText), so user-facing
error text is localized instead of substring-matched from English blobs.
.raw(String) remains only for genuinely dynamic system/core messages.

Localize the CoreNFC alertMessage prompts via existing L10n keys, and add
SwiftLint rules (raw_alert_message, raw_invitation_error) to catch raw
alert strings and literal .raw("…") errors going forward.

apple/VniDrop/Core/CoreRepository.swift

@@ -156,7 +156,7 @@ final class CoreRepository: ObservableObject, CoreGateway {
 			return .failure(CoreNetworkLifecycleError.transitionInProgress)
 		}
 		guard !sources.isEmpty else {
-			return .failure(InvitationError.message("Select at least one file to share"))
+			return .failure(InvitationError.shareEmpty)
 		}
 		return await runCore {
 			let result = try self.requireCore().shareFiles(
@@ -353,7 +353,7 @@ final class CoreRepository: ObservableObject, CoreGateway {
 
 	private nonisolated func requireCore() throws -> VnidropCore {
 		guard let core = self.core else {
-			throw InvitationError.message("Initialize the core first.")
+			throw InvitationError.coreNotInitialized
 		}
 		return core
 	}