Add 6 desktop features: workspace shortcuts, Exposé, deskbar icons, notifications, screen lock, move-to-desktop

- Super+1-4: switch virtual desktop; Super+Shift+1-4: move focused window to desktop N
- Super+Space: Exposé window overview (tiled grid), click to select, Escape to cancel
- wm/src/expose.c: expose_show/expose_hide using ceil(sqrt) grid layout with saved/restored geometry
- client_move_to_desktop(): moves window between desktops, fires _NET_WM_DESKTOP update
- on_client_message: handles _NET_WM_DESKTOP to move windows from external requests
- deskbar: optional imlib2 app icon rendering (Papirus-Dark → Papirus → hicolor → pixmaps fallback)
- deskbar: XGetClassHint populates cls field for better icon lookup
- deskbar: context menu extended with separator + "→ Desktop 1-4" items via _NET_WM_DESKTOP
- orionnotify/: freedesktop.org notification daemon (libdbus-1), stacks up to 5 popups top-right
- orionlock/: PAM-authenticated fullscreen screen lock with clock, date, password dots
- Lock Screen menu item wired to orionlock
- Root Makefile includes orionnotify and orionlock targets

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GHiBQKPfUoaJgCKuA9op1H

wm/Makefile

@@ -12,9 +12,9 @@ ifeq ($(PKG_XFT_C),)
 endif
 
 CFLAGS  = -Wall -Wextra -O2 -Iinclude $(PKG_XFT_C)
-LDFLAGS = $(PKG_XFT_L)
+LDFLAGS = $(PKG_XFT_L) -lm
 
-SRC = src/wm.c src/client.c src/events.c src/draw.c src/color.c src/menu.c src/switcher.c
+SRC = src/wm.c src/client.c src/events.c src/draw.c src/color.c src/menu.c src/switcher.c src/expose.c
 OBJ = $(SRC:.c=.o)
 BIN = orionwm