feat(gui): add a GTK sender panel and a waybar widget

A gtkmm-4.0 control panel (behind -Dgui=true, default off): refresh
shows discovered receivers (grouped and preference-sorted), a bitrate
scale, and start/stop that runs the whole session on a worker thread
so the interactive portal picker never blocks the UI. The CLI and the
GUI now share the new sc_app_core static library holding the
pipelines, session orchestration (negotiation + PLI feedback), and a
state store.

The sender pipeline publishes its state to
$XDG_RUNTIME_DIR/screencast/sender.json (session id, receiver,
bitrate, pid, start time; stale files detected by pid liveness) and
persists the last session for one-click restarts. The new
'screencast waybar' subcommand prints a waybar module line and its
--toggle flag stops a running sender gracefully or spawns a detached
restart of the last receiver.

Waybar on the dev machine is wired: custom/screencast module with
click-to-toggle and right-click panel, plus styles, with a timestamped
backup of both config files. Both binaries are installed to
/usr/local/bin.

Validated: waybar output (idle and streaming states with a synthetic
state file), GUI launches on the desktop (window observed via
hyprctl), meson test 5/5 in both build configurations, formatting
clean.

src/app/cli.cpp

@@ -11,6 +11,7 @@ void print_usage() {
     std::fputs("usage: screencast --send [--target monitor|window] [--peer HOST[:PORT]] [--bitrate KBPS]\n"
                "       screencast --receive [--port PORT] [--signaling-port PORT] [--fullscreen] [--swdecode]\n"
                "       screencast --discover [--timeout SECONDS]\n"
+               "       screencast waybar [--toggle]        # for waybar widgets\n"
                "\n"
                "--send without --peer discovers a receiver on the LAN and requires\n"
                "that exactly one is found.\n",
@@ -39,12 +40,14 @@ std::optional<Command> parse_cli(int argc, const char* const argv[]) {
         Send,
         Receive,
         Discover,
+        Waybar,
     };
 
     Mode mode = Mode::None;
     SendCommand send;
     ReceiveCommand receive;
     DiscoverCommand discover;
+    WaybarCommand waybar;
 
     for (int index = 1; index < argc; ++index) {
         const std::string_view argument = argv[index];
@@ -67,6 +70,14 @@ std::optional<Command> parse_cli(int argc, const char* const argv[]) {
                 return std::nullopt;
             }
             mode = Mode::Discover;
+        } else if (argument == "waybar") {
+            if (mode != Mode::None) {
+                print_usage();
+                return std::nullopt;
+            }
+            mode = Mode::Waybar;
+        } else if (argument == "--toggle") {
+            waybar.toggle = true;
         } else if (argument == "--target") {
             std::string_view value;
             if (!next_argument(argc, argv, index, value)) {
@@ -136,6 +147,9 @@ std::optional<Command> parse_cli(int argc, const char* const argv[]) {
     if (mode == Mode::Discover) {
         return Command{std::move(discover)};
     }
+    if (mode == Mode::Waybar) {
+        return Command{std::move(waybar)};
+    }
     print_usage();
     return std::nullopt;
 }