ask which source to open when --server is absent

The chooser runs before ScopeModel exists, so declining costs no buffer and no
socket. UDPSCOPE_PROBE_SOURCES=1 prints what the discovery found and exits,
before gtk_init(), so the list is checkable with no display.

src/source_dialog.hpp

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#pragma once
+
+#include "config.hpp"
+
+namespace udpscope {
+
+/// Asks which data source to open: a list of the sources with a stored session, and a form for a
+/// new one. Returns true with `cfg`'s connection fields filled in, or false if the user declined
+/// -- in which case `cfg` is untouched and the caller should exit.
+///
+/// Runs *before* `ScopeModel` exists, so declining costs no buffer and no socket. It drives its
+/// own `gtk_init()` and nested `GMainLoop`; `run_ui()`'s `GtkApplication` comes afterwards in the
+/// same process, which works because `gtk_init()` is idempotent.
+bool choose_source(Config& cfg);
+
+}  // namespace udpscope