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.

include/screencast/app/cli.h

@@ -25,7 +25,11 @@ struct DiscoverCommand {
     int timeout_seconds = 3;
 };
 
-using Command = std::variant<SendCommand, ReceiveCommand, DiscoverCommand>;
+struct WaybarCommand {
+    bool toggle = false; // toggle streaming instead of printing status
+};
+
+using Command = std::variant<SendCommand, ReceiveCommand, DiscoverCommand, WaybarCommand>;
 
 // Parse command line arguments. Prints usage and returns std::nullopt on error.
 // `argv` is `char const* const*` so both `main`'s `char**` and const arrays

include/screencast/app/pipeline.h

@@ -19,6 +19,9 @@ struct SenderPipelineConfig {
     // Where encoded RTP packets are sent. Defaults to the local loopback so
     // a sender and receiver on one machine work without any configuration.
     Endpoint peer_rtp_endpoint{"127.0.0.1", 5004};
+    // Session identity from signaling; written to the sender state file for
+    // status widgets (waybar) and one-click restarts.
+    std::string session_id;
 };
 
 struct ReceiverPipelineConfig {