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).

wm/include/wm.h

@@ -73,10 +73,15 @@ typedef struct Client {
     int focused;
     int minimized;
     int maximized;
+    int desktop;               /* virtual desktop this window lives on */
     int saved_x, saved_y;      /* pre-maximize position */
     unsigned saved_w, saved_h; /* pre-maximize size */
     Time last_click_time;      /* for double-click detection */
     char title[256];
+    /* ICCCM WM_NORMAL_HINTS */
+    int min_w, min_h;          /* PMinSize */
+    int inc_w, inc_h;          /* PResizeInc — 0 means no constraint */
+    int base_w, base_h;        /* PBaseSize — baseline for increment calc */
     struct Client *next;
 } Client;
 
@@ -104,6 +109,8 @@ typedef struct WM {
     Client *clients;
     Client *focused;
 
+    int  cur_desktop;          /* active virtual desktop (0-based) */
+
     Atom wm_delete_window;
     Atom wm_protocols;
     Atom wm_state;
@@ -112,6 +119,8 @@ typedef struct WM {
     Atom net_wm_state_hidden;
     Atom net_wm_state_maximized_vert;
     Atom net_wm_state_maximized_horz;
+    Atom net_wm_desktop;
+    Atom net_current_desktop;
 
     DragMode drag_mode;
     int      drag_start_x, drag_start_y;