test: cover message quote formatting and draft append behavior

Pinned multiline quotes, system-sender omission, and non-destructive append.

bitchatTests/Services/MessageClipboardTests.swift

@@ -0,0 +1,37 @@
+import Foundation
+import Testing
+@testable import bitchat
+
+struct MessageClipboardTests {
+    @Test func quoteWrapsEachLineAndAddsSender() {
+        let quoted = MessageClipboard.quoteForComposer("hello\nworld", sender: "alice")
+        #expect(quoted == "> @alice:\n> hello\n> world\n\n")
+    }
+
+    @Test func quoteSkipsSystemSenderHeader() {
+        let quoted = MessageClipboard.quoteForComposer("joined", sender: "system")
+        #expect(quoted == "> joined\n\n")
+    }
+
+    @Test func emptyContentYieldsEmptyQuote() {
+        #expect(MessageClipboard.quoteForComposer("   ", sender: "alice").isEmpty)
+    }
+
+    @Test func appendQuotePreservesExistingDraft() {
+        let result = MessageClipboard.appendQuote(
+            to: "already typing",
+            content: "prior message",
+            sender: "bob"
+        )
+        #expect(result == "already typing\n> @bob:\n> prior message\n\n")
+    }
+
+    @Test func appendQuoteOntoEmptyDraft() {
+        let result = MessageClipboard.appendQuote(
+            to: "",
+            content: "solo",
+            sender: "carol"
+        )
+        #expect(result == "> @carol:\n> solo\n\n")
+    }
+}