fix(forward): warn when forwarding out of an encrypted room into one that is not (#330)

Forwarding from an encrypted room into an unencrypted one posts the text
(and a re-uploaded plaintext attachment) where the server can read it,
with the original author attached, and the dialog said nothing. Show a
note above Send naming the unencrypted destination(s).

Refs #330

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/features/room/message/forwardContent.ts

@@ -122,3 +122,23 @@ export async function buildPlaintextAttachmentContent(
 
   return plainContent;
 }
+
+/**
+ * [Gitea #330] The note shown when forwarding out of an encrypted room into
+ * rooms that aren't: the text (and a re-uploaded attachment) becomes readable
+ * by the server, and the original author is shown with it. Undefined when
+ * there's nothing to warn about.
+ */
+export function forwardEncryptionWarning(
+  sourceEncrypted: boolean,
+  unencryptedDestNames: string[],
+): string | undefined {
+  if (!sourceEncrypted || unencryptedDestNames.length === 0) return undefined;
+  const where =
+    unencryptedDestNames.length === 1
+      ? `${unencryptedDestNames[0]} isn't encrypted.`
+      : `${unencryptedDestNames.length} of the rooms you picked aren't encrypted.`;
+  return `${where} The forwarded message (and any attachment) will be readable by the server and everyone in ${
+    unencryptedDestNames.length === 1 ? 'that room' : 'those rooms'
+  }, along with who originally wrote it.`;
+}