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/Makefile

@@ -14,7 +14,7 @@ endif
 CFLAGS  = -Wall -Wextra -O2 -Iinclude $(PKG_XFT_C)
 LDFLAGS = $(PKG_XFT_L)
 
-SRC = src/wm.c src/client.c src/events.c src/draw.c src/color.c src/menu.c
+SRC = src/wm.c src/client.c src/events.c src/draw.c src/color.c src/menu.c src/switcher.c
 OBJ = $(SRC:.c=.o)
 BIN = skywm