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/gui/meson.build

@@ -0,0 +1,15 @@
+# GTK4 sender panel, behind the `gui` option (default: false). The core
+# CLI and tests never need GTK.
+
+dep_gtkmm = dependency('gtkmm-4.0')
+
+screencast_gui_sources = files(
+    'gui.cpp',
+)
+
+executable('screencast-gui',
+    screencast_gui_sources,
+    include_directories : [sc_core_inc, include_directories('../app')],
+    dependencies : [dep_gtkmm, sc_app_core_dep, sc_capture_dep, sc_codec_dep, sc_network_dep,
+                   sc_render_dep],
+    install : true)