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.

docs/RUNBOOK.md

@@ -134,6 +134,24 @@ A window manager only comes with the desktop-session alternative, where
 the receiver runs inside it (uncomment the `Environment=` lines in the
 service file as described above).
 
+## GTK panel and waybar widget (Phase 7)
+
+Build the GUI alongside the CLI (`meson configure build -Dgui=true`, then
+recompile; installs as `screencast-gui`):
+
+```sh
+screencast-gui          # receiver list → pick one → bitrate → Start
+screencast waybar       # one JSON line for a waybar custom module
+screencast waybar --toggle   # stop a running sender / restart the last
+```
+
+The waybar module (installed to this machine's `~/.config/waybar/` — see
+`custom/screencast` there): `▶` while streaming (tooltip: receiver, bitrate,
+elapsed), `⏸` while idle; left-click toggles streaming to the last receiver
+via the state file, right-click opens the panel. The sender publishes its
+state to `$XDG_RUNTIME_DIR/screencast/sender.json`, so every front-end —
+CLI, GUI, widget — agrees on what is running.
+
 ## Under the hood: resilience (Phase 7)
 
 The receiver absorbs loss in two stages and recovers actively: