feat: preferences for remote clipboard writes, multi-line paste confirmation and the launch update check

OSC 52 and VNC ServerCutText now go through one handler that honours
'allow remote clipboard' and always toasts the first line, so a silent
clipboard swap can't set up a paste into another shell (SEC-20261012-010).
Multi-line pastes into a terminal without bracketed paste ask first, with a
preview (SEC-20261012-017). The launch-time update check can be turned off
and the updater docs no longer claim it is opt-in; the unused GitHub
fallback is removed (SEC-20261012-013, F15).

rust/app/src/sftp.rs

@@ -1122,7 +1122,8 @@ fn paste_files(s: &mut Shared, session: &str, paths: &[PathBuf]) {
     }
     if let Some(View::Terminal(term)) = s.panes.get_mut(session).map(|p| &mut p.view) {
         let mut out = crate::terminal::TermResponse::default();
-        term.paste(&combined, &mut out);
+        // The drop was already confirmed in its own dialog.
+        term.paste_now(&combined, &mut out);
         s.request(Request::Write(session.into(), out.input));
     }
 }