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.

docs/PHASES.md

@@ -89,9 +89,10 @@ discovered addresses in reachability order (private IPv4 first).
 - [x] Jitter/reorder window on the receiver (16 packets / 60 ms).
 - [x] Hardware H.264 decode probe with software fallback (h264_v4l2m2m —
       the Pi's VideoCore path; --swdecode opts out).
-- [ ] GTK4 sender GUI behind `meson -Dgui=true` + a waybar widget
-      (requested; plan approved scope: sender panel, state file,
-      `screencast waybar` subcommand).
+- [x] GTK4 sender GUI behind `meson -Dgui=true` (gtkmm; receiver list,
+      bitrate, start/stop) plus a waybar widget (`screencast waybar`,
+      click-to-toggle, right-click opens the panel; state shared via
+      $XDG_RUNTIME_DIR/screencast/sender.json).
 - [ ] VAAPI hardware encode probe on the sender (deferred: software
       encode is not the bottleneck).
 - Deferred: .desktop file, packaging.