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.

.agents/MEMORY.md

@@ -65,7 +65,17 @@ loopback; current phase is Phase 7.
   before the fix). Cross-compiling on the dev machine was considered and
   dropped — the on-Pi build works and the toolchain/container effort was
   not needed.
-- **Pi Wi-Fi hotspot** (`scripts/pi-hotspot.sh on|off|status`): NetworkManager
+- **GTK panel + waybar widget**: `screencast-gui` (gtkmm-4.0, behind
+  `-Dgui=true`; app internals now live in the `sc_app_core` static lib so
+  CLI and GUI share pipelines/session/state). `screencast waybar [--toggle]`
+  prints the waybar module line and toggles streaming via SIGTERM (stop) or
+  a detached re-exec spawn from `last-session.json` (start). State lives in
+  `$XDG_RUNTIME_DIR/screencast/sender.json` (written by the pipeline with
+  session id, receiver, bitrate, pid, start time; stale files are detected
+  by pid-liveness). The waybar config was wired into the user's bar
+  (`custom/screencast` before `custom/timetrack`, with backup) and the
+  binaries installed to /usr/local/bin.
+  - **Pi Wi-Fi hotspot** (`scripts/pi-hotspot.sh on|off|status`): NetworkManager
   AP mode (WPA2, ipv4 shared → built-in DHCP/NAT, Pi at 10.42.0.1). Takes
   over wlan0 while active; generated PSK stored in /etc/screencast-hotspot.conf.
   No application changes needed — the receiver already announces on all