feat: add system monitor feature with panel toggle and event handling

- Introduce SystemMonitor struct and typedef in dwn.h, add sysmon pointer to Monitor struct
- Include sysmon.h in main.c, integrate sysmon_find_by_window and sysmon_set_client into map request handling
- Add sysmon_render call in expose handler and sysmon_handle_event in button press and motion notify handlers
- Implement clickable stats and top process areas in bottom panel to toggle system monitor via sysmon_toggle
- Update build/main.d dependency file to include sysmon.h

build/main.d

@@ -21,7 +21,7 @@ build/main.o: src/main.c include/dwn.h include/config.h include/dwn.h \
  /usr/include/dbus-1.0/dbus/dbus-threads.h include/systray.h \
  include/news.h include/applauncher.h include/ai.h include/autostart.h \
  include/services.h include/api.h include/demo.h include/screenshot.h \
- include/ocr.h include/util.h
+ include/ocr.h include/sysmon.h include/util.h
 include/dwn.h:
 include/config.h:
 include/dwn.h:
@@ -61,4 +61,5 @@ include/api.h:
 include/demo.h:
 include/screenshot.h:
 include/ocr.h:
+include/sysmon.h:
 include/util.h: