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

include/dwn.h

@@ -107,6 +107,7 @@ typedef struct Workspace Workspace;
 typedef struct Monitor Monitor;
 typedef struct Panel Panel;
 typedef struct Config Config;
+typedef struct SystemMonitor SystemMonitor;
 
 struct Client {
     Window window;
@@ -176,6 +177,8 @@ typedef struct {
     Panel *top_panel;
     Panel *bottom_panel;
 
+    SystemMonitor *sysmon;
+
     Config *config;
 
     bool running;