macOS-style polish: fix seam, bidirectional Alt+Tab, minimized windows in switcher, rounded corners, deskbar context menu

Fix #1: Replace hardcoded 185px deskbar-offset in client.c and events.c with
DESKBAR_W constant (200) to eliminate the 15px gap between the WM frame and
deskbar edge.

Fix #2: Add Shift+Alt+Tab reverse-direction support — grab Mod1+Shift+XK_Tab
in wm.c, detect ShiftMask in events.c, and implement switcher_prev() in
switcher.c.

Fix #3: Include minimized windows in the Alt+Tab switcher. swl_populate() no
longer skips minimized clients; swl_mini[] tracks state so switcher_draw()
can dim them with bracketed titles and darker card backgrounds.

Fix #4: Rewrite themes/picom.conf with macOS-inspired settings — 10px corner
radius on normal windows, 8px on menus, soft downward shadow (radius 18,
offset-y 8, opacity 0.35), smooth fading (step 0.08), no dock/tray rounding.

Fix #5: Add per-app right-click context menu to the deskbar. Right-clicking
an app row opens a popup with Raise / Minimize / Close actions (Close shown
in red). Actions are sent as EWMH client messages (_NET_ACTIVE_WINDOW,
WM_CHANGE_STATE+IconicState, _NET_CLOSE_WINDOW) with matching handlers added
to the WM's on_client_message. Pointer grab closes the menu on any outside
click.

wm/include/wm.h

@@ -29,6 +29,7 @@
 #define COLOR_SNAP_OVERLAY     0x4A90D9   /* macOS blue snap highlight */
 
 /* ── geometry ────────────────────────────────────────────────────────── */
+#define DESKBAR_W      200         /* must match DB_W in deskbar/include/deskbar.h */
 #define TITLE_HEIGHT   28          /* macOS-style height */
 #define TITLE_TAB_W    200
 #define BORDER_W       1           /* thin 1px macOS-style border */
@@ -130,6 +131,7 @@ typedef struct WM {
     int    swl_count;
     Window swl_wins[64];
     char   swl_titles[64][128];
+    int    swl_mini[64];           /* 1 if corresponding entry is minimized */
 } WM;
 
 /* core */
@@ -175,6 +177,7 @@ int  menu_item_at(int py);
 /* switcher */
 void switcher_show(WM *wm);
 void switcher_next(WM *wm);
+void switcher_prev(WM *wm);
 void switcher_hide(WM *wm, int commit);
 void switcher_draw(WM *wm);