Fixes #7–#11: launcher Other tab, ICCCM size hints, icon loading, multi-desktop

Fix #7: Add "Other" catch-all category to the launcher. map_category() now
returns "Other" instead of "" for unrecognized .desktop Categories, and an
"Other" tab is added to the CATS list so those apps are discoverable instead
of silently hidden.

Fix #8: Respect ICCCM WM_NORMAL_HINTS. client_add() reads PMinSize,
PResizeInc, and PBaseSize from the window's size hints. The resize drag
handler in events.c clamps to min_w/min_h and snaps to the
width_inc/height_inc grid, fixing jank in character-cell terminal emulators
(xterm, Alacritty).

Fix #10: Optional freedesktop icon loading via imlib2. The launcher now
parses the Icon= field from .desktop files and attempts to render a PNG icon
from Papirus-Dark (or hicolor fallback). When imlib2 is not present the
existing letter-square is shown unchanged. launcher/Makefile auto-detects
imlib2 via pkg-config and sets -DHAVE_IMLIB2.

Fix #11: True 4-desktop support in the WM. Each Client carries a desktop
field (assigned at map time from wm->cur_desktop). Clicking a workspace
button in the deskbar sends _NET_CURRENT_DESKTOP; the WM now handles it via
wm_switch_desktop() which unmaps frames on the leaving desktop, maps frames
on the arriving desktop, and re-focuses. _NET_WM_DESKTOP is set per window
so the deskbar can filter db_update_apps() to show only current-desktop
windows (sticky windows with desktop 0xFFFFFFFF are always shown).

launcher/Makefile

@@ -7,6 +7,14 @@ ifeq ($(PKG_XFT_C),)
   PKG_XFT_L := -L/opt/homebrew/lib -lXft -lfontconfig
 endif
 
+# Optional: imlib2 for freedesktop icon loading
+PKG_IL_C := $(shell pkg-config --cflags imlib2 2>/dev/null)
+PKG_IL_L := $(shell pkg-config --libs   imlib2 2>/dev/null)
+ifneq ($(PKG_IL_C),)
+  PKG_XFT_C += $(PKG_IL_C) -DHAVE_IMLIB2
+  PKG_XFT_L += $(PKG_IL_L)
+endif
+
 CFLAGS  = -Wall -Wextra -O2 $(PKG_XFT_C)
 LDFLAGS = $(PKG_XFT_L)