feat(gui): Add SUB-WT widget toolkit, icon set, modal dialogs, six apps and a reworked window manager

Turns the desktop from a fixed set of hand-drawn panels into a real
environment applications can be written against.

SUB-WT immediate-mode toolkit (gui/gui_widgets.c):
  Buttons, labels, checkboxes, radios, sliders, list boxes, text fields, tab
  bars, scrollbars, progress bars, sparklines and badges. An app feeds the WM
  event into the toolkit and then calls widgets from its paint routine, so
  each call both draws the control and reports what the user did with it --
  no retained widget tree to keep in sync with window state.

Icons and dialogs:
  gui_icons.c holds 14 16x16 palette-indexed glyphs stored as hex text, so the
  art stays hand-editable and picks up theme changes for free; they scale by
  integer factors and can be drawn as tint stencils. gui_dialog.c adds modal
  info/warning/error/confirm dialogs that dim the desktop, take exclusive
  input, word-wrap on spaces and report results through a callback.

Applications (gui/gui_apps_ext.c):
  Settings (tabbed appearance/input/power), Task Manager (live process table
  over task_find_by_pid with memory and heap gauges), Kernel Log viewer (reads
  the dmesg ring, strips ANSI, colours by severity, scrolls), KTest Runner
  (drives the in-kernel suites and reports 21/21 with timing), Clock & Calendar
  (Zeller's congruence month grid) and a Text Editor.

Window manager (gui/gui_wm.c):
  Minimize/maximize/restore buttons, a corner resize grip, edge snapping, tile
  and cascade layouts, F5-F9 shortcuts, title truncation, and index-based
  enumeration so the taskbar no longer assumes window ids stay dense.

Desktop shell (gui/gui_desktop.c):
  Launcher icon grid, richer start menu, right-click context menu, taskbar
  buttons that minimize and restore, and a live CPU trace in the tray.

Two interaction bugs fixed along the way:
  - keyboard_get_key() blocks until a key arrives, so calling it every frame
    froze the compositor; it is now gated on keyboard_has_key(), and stale
    console input is drained before the first frame.
  - When the desktop consumed a click, the WM's press-edge detector kept the
    pre-press state and read the still-held button as a fresh press on the
    next frame, stealing focus from whatever the click had just opened.
    gui_wm_sync_button_state() keeps the two layers in step.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01L5yKJT4Wo2cV3rfPrwY5MP

include/gui/gui_wm.h

@@ -7,8 +7,11 @@
 #include <gui/gui_gfx.h>
 #include <gui/gui_theme.h>
 
-#define GUI_MAX_WINDOWS 16
-#define GUI_TITLEBAR_HEIGHT 24
+#define GUI_MAX_WINDOWS      16
+#define GUI_TITLEBAR_HEIGHT  24
+#define GUI_RESIZE_GRIP      12
+#define GUI_MIN_WINDOW_W     160
+#define GUI_MIN_WINDOW_H     100
 
 typedef enum {
     GUI_EVENT_MOUSE_MOVE,
@@ -16,6 +19,7 @@ typedef enum {
     GUI_EVENT_MOUSE_UP,
     GUI_EVENT_KEY_DOWN,
     GUI_EVENT_FOCUS,
+    GUI_EVENT_RESIZE,
     GUI_EVENT_CLOSE
 } gui_event_type_t;
 
@@ -30,19 +34,40 @@ typedef struct {
     uint16_t key;
 } gui_event_t;
 
+// Where a window sits relative to the screen edges.
+typedef enum {
+    GUI_SNAP_NONE = 0,
+    GUI_SNAP_LEFT,
+    GUI_SNAP_RIGHT,
+    GUI_SNAP_MAXIMIZED
+} gui_snap_t;
+
 typedef struct gui_window {
-    int id;
+    int  id;
     char title[64];
-    int x;
-    int y;
-    int width;
-    int height;
+    int  x;
+    int  y;
+    int  width;
+    int  height;
+
     bool active;
     bool minimized;
     bool visible;
+    bool resizable;
+
+    // Drag / resize interaction state
     bool is_dragging;
-    int drag_off_x;
-    int drag_off_y;
+    bool is_resizing;
+    int  drag_off_x;
+    int  drag_off_y;
+
+    // Geometry remembered across maximize / snap so restore is exact
+    gui_snap_t snap;
+    int  saved_x;
+    int  saved_y;
+    int  saved_width;
+    int  saved_height;
+
     void* user_data;
 
     void (*paint)(struct gui_window* win);
@@ -50,12 +75,34 @@ typedef struct gui_window {
 } gui_window_t;
 
 void gui_wm_init(void);
+
+// The work area excludes the taskbar; windows maximize and snap inside it.
+void gui_wm_set_workarea(int width, int height);
+
 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);
+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);
+int           gui_wm_get_window_count(void);
+
+// Enumerate in z-order, bottom-most first. Returns NULL past the end.
+gui_window_t* gui_wm_get_window_at_index(int index);
+
+void gui_wm_minimize_window(int win_id);
+void gui_wm_restore_window(int win_id);
+void gui_wm_toggle_maximize(int win_id);
+void gui_wm_snap_window(int win_id, gui_snap_t snap);
+void gui_wm_close_active(void);
+void gui_wm_cycle_focus(void);
+void gui_wm_cascade_windows(void);
+void gui_wm_tile_windows(void);
+
+// Keep the WM's press-edge detector in step when another layer (a menu, a
+// modal dialog) consumed the click instead. Without this the WM still holds
+// the pre-press state and reads the still-held button as a fresh press on the
+// following frame, stealing focus from whatever the click just opened.
+void gui_wm_sync_button_state(bool btn_left);
 
 void gui_wm_handle_mouse(int x, int y, bool btn_left, bool btn_right);
 void gui_wm_handle_key(uint16_t key);