feat(gui): Implement complete Graphical Desktop Environment, SUB-WM, and apps

- Create separate feature branch 'feat/gui-desktop'
- Implement double-buffered 2D graphics engine with alpha blending, gradients, rounded rects, and font rasterization in gui/gui_gfx.c and include/gui/gui_gfx.h
- Implement alpha-blended mouse pointer overlay in gui/gui_cursor.c and include/gui/gui_cursor.h
- Implement SUB-WM Window Manager with draggable titlebars, close/minimize buttons, drop shadows, and event routing in gui/gui_wm.c and include/gui/gui_wm.h
- Implement Cyberpunk/Nord desktop environment with wallpaper, taskbar, start menu popup, and system tray in gui/gui_desktop.c and include/gui/gui_desktop.h
- Implement built-in GUI apps: System Performance Monitor, GUI Terminal Emulator, File Explorer, Calculator, Paint Studio, and About dialog in gui/gui_apps.c and include/gui/gui_apps.h
- Add 'startx', 'gui', and 'desktop' applets in LazyBox suite
- Verified multi-architecture compilation across x86_64, AArch64, and ARMv8i

include/gui/gui_wm.h

@@ -0,0 +1,64 @@
+#ifndef _GUI_WM_H
+#define _GUI_WM_H
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <gui/gui_gfx.h>
+#include <gui/gui_theme.h>
+
+#define GUI_MAX_WINDOWS 16
+#define GUI_TITLEBAR_HEIGHT 16
+
+typedef enum {
+    GUI_EVENT_MOUSE_MOVE,
+    GUI_EVENT_MOUSE_DOWN,
+    GUI_EVENT_MOUSE_UP,
+    GUI_EVENT_KEY_DOWN,
+    GUI_EVENT_FOCUS,
+    GUI_EVENT_CLOSE
+} gui_event_type_t;
+
+typedef struct {
+    gui_event_type_t type;
+    int mouse_x;
+    int mouse_y;
+    int rel_x;
+    int rel_y;
+    bool btn_left;
+    bool btn_right;
+    uint16_t key;
+} gui_event_t;
+
+typedef struct gui_window {
+    int id;
+    char title[48];
+    int x;
+    int y;
+    int width;
+    int height;
+    bool active;
+    bool minimized;
+    bool visible;
+    bool is_dragging;
+    int drag_off_x;
+    int drag_off_y;
+    void* user_data;
+
+    void (*paint)(struct gui_window* win);
+    void (*handle_event)(struct gui_window* win, const gui_event_t* ev);
+} gui_window_t;
+
+void gui_wm_init(void);
+gui_window_t* gui_wm_create_window(const char* title, int x, int y, int w, int h);
+void gui_wm_destroy_window(int win_id);
+void gui_wm_focus_window(int win_id);
+gui_window_t* gui_wm_get_window(int win_id);
+gui_window_t* gui_wm_get_active_window(void);
+int gui_wm_get_window_count(void);
+
+void gui_wm_handle_mouse(int x, int y, bool btn_left, bool btn_right);
+void gui_wm_handle_key(uint16_t key);
+void gui_wm_render(void);
+
+#endif // _GUI_WM_H