Phase 1: Alt+Tab switcher, dark GTK theme, alacritty terminal

- Alt+Tab now shows a centered visual switcher overlay with window cards;
  cycling with Tab, committing on Alt release, cancelling with Escape
- GTK3 and GTK2 themes rewritten in dark navy palette matching skywm/
  skydeskbar color constants (was an off-brand tan/BeOS palette)
- Add alacritty to ISO packages with pre-configured dark color scheme;
  update all menu Terminal entries to launch alacritty instead of xterm
- Add KeyReleaseMask to WM root event selection to detect Alt release

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

wm/include/wm.h

@@ -121,6 +121,14 @@ typedef struct WM {
     int    menu_visible;
     int    menu_x, menu_y;
     int    menu_hovered;
+
+    /* Alt+Tab visual switcher */
+    Window swl_win;
+    int    swl_visible;
+    int    swl_idx;
+    int    swl_count;
+    Window swl_wins[64];
+    char   swl_titles[64][128];
 } WM;
 
 /* core */
@@ -163,6 +171,12 @@ void menu_draw(WM *wm);
 void menu_click(WM *wm, int px, int py);
 int  menu_item_at(int py);
 
+/* switcher */
+void switcher_show(WM *wm);
+void switcher_next(WM *wm);
+void switcher_hide(WM *wm, int commit);
+void switcher_draw(WM *wm);
+
 /* events */
 void on_map_request(WM *wm, XMapRequestEvent *e);
 void on_unmap_notify(WM *wm, XUnmapEvent *e);
@@ -176,5 +190,6 @@ void on_key_press(WM *wm, XKeyEvent *e);
 void on_enter_notify(WM *wm, XCrossingEvent *e);
 void on_client_message(WM *wm, XClientMessageEvent *e);
 void on_property_notify(WM *wm, XPropertyEvent *e);
+void on_key_release(WM *wm, XKeyEvent *e);
 
 #endif