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.cpp

@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: MIT
+// SPDX-FileCopyrightText: 2026 Martino Ferrari
+
+#include "source_dialog.hpp"
+
+#include "layout_store.hpp"
+
+#include <gtk/gtk.h>
+
+#include <cstdio>
+#include <string>
+#include <vector>
+
+namespace udpscope {
+namespace {
+
+/// A chooser row's source becomes the connection to open. The interface comes along with the
+/// group: it is the local address the join happens on, so storing one without the other would
+/// reconnect to the right group on the wrong path.
+[[maybe_unused]] void apply_source(Config& cfg, const KnownSource& src) {
+  cfg.server = src.host;
+  cfg.port = src.port;
+  cfg.multicast = src.multicast;
+  cfg.interface_addr = src.interface_addr;
+}
+
+/// One line per known source on stderr, in the style of the probes in `ui.cpp`.
+void report_sources(const std::vector<KnownSource>& known) {
+  std::fprintf(stderr, "[sources] %zu known:", known.size());
+  for (const KnownSource& s : known) {
+    std::fprintf(stderr, " %s:%u plots=%d signals=%d", s.host.c_str(), s.port, s.rows, s.signals);
+    if (!s.multicast.empty()) std::fprintf(stderr, " multicast=%s", s.multicast.c_str());
+    if (!s.interface_addr.empty()) {
+      std::fprintf(stderr, " interface=%s", s.interface_addr.c_str());
+    }
+    std::fprintf(stderr, " |");
+  }
+  std::fprintf(stderr, "\n");
+  std::fflush(stderr);
+}
+
+}  // namespace
+
+bool choose_source(Config& cfg) {
+  const std::vector<KnownSource> known = known_sources();
+
+  // The discovery half, checkable with no display at all: this returns before `gtk_init()`, so
+  // it asserts what the list found without needing an X server or a GL context. False means the
+  // caller exits 0, which is what "list and exit" wants.
+  if (g_getenv("UDPSCOPE_PROBE_SOURCES") != nullptr) {
+    report_sources(known);
+    return false;
+  }
+
+  gtk_init();
+  (void)cfg;
+  return false;
+}
+
+}  // namespace udpscope