feat: add plaintext chat exporter with sealing warning header

Format on-device messages as a plain-text transcript and prepend an
explicit warning that exported files are unsealed and readable by anyone
with a copy.

bitchat/Utils/ChatPlaintextExporter.swift

@@ -0,0 +1,54 @@
+import BitFoundation
+import Foundation
+
+/// Formats on-device chat history as unsealed plaintext for export.
+enum ChatPlaintextExporter {
+    struct Context: Equatable {
+        let scopeTitle: String
+        let exportedAt: Date
+
+        init(scopeTitle: String, exportedAt: Date = Date()) {
+            self.scopeTitle = scopeTitle
+            self.exportedAt = exportedAt
+        }
+    }
+
+    static func exportText(messages: [BitchatMessage], context: Context) -> String {
+        let header = [
+            "bitchat export — \(context.scopeTitle)",
+            "exported \(exportTimestamp(context.exportedAt))",
+            "",
+            warningLine,
+            "",
+        ].joined(separator: "\n")
+
+        guard !messages.isEmpty else {
+            return header + String(localized: "chat.export.empty", comment: "Line shown when an export contains no messages")
+        }
+
+        let body = messages.map { line(for: $0) }.joined(separator: "\n")
+        return header + body + "\n"
+    }
+
+    private static var warningLine: String {
+        String(
+            localized: "chat.export.sealing_warning",
+            comment: "Warning that an exported chat file is unsealed plaintext readable by anyone with a copy"
+        )
+    }
+
+    private static func line(for message: BitchatMessage) -> String {
+        let stamp = exportTimestamp(message.timestamp)
+        let sender = message.sender.trimmingCharacters(in: .whitespacesAndNewlines)
+        let content = message.content.trimmingCharacters(in: .whitespacesAndNewlines)
+        return "[\(stamp)] \(sender): \(content)"
+    }
+
+    private static func exportTimestamp(_ date: Date) -> String {
+        let formatter = DateFormatter()
+        formatter.locale = Locale(identifier: "en_US_POSIX")
+        formatter.timeZone = TimeZone.current
+        formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
+        return formatter.string(from: date)
+    }
+}