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.

deskbar/include/deskbar.h

@@ -44,6 +44,12 @@
 #define DB_TRAY_MAX    16     /* maximum simultaneous tray icons */
 #define DB_MAX_APPS    32
 
+/* ── per-app context menu ────────────────────────────────────────────── */
+#define DB_CTX_W       140     /* context menu width */
+#define DB_CTX_ITEM_H   26     /* item row height */
+#define DB_CTX_PAD       4     /* top/bottom padding */
+#define DB_CTX_N         3     /* number of items: Raise, Minimize, Close */
+
 /* ── fly-out menu ────────────────────────────────────────────────────── */
 #define DB_FLYOUT_W    196     /* fly-out menu panel width */
 #define DB_FLYOUT_IH    28     /* item row height */
@@ -139,6 +145,11 @@ typedef struct Deskbar {
     int       sub_hov;      /* hovered item in sub-menu */
     int       sw_cache;     /* cached screen width for menu positioning */
 
+    /* per-app right-click context menu */
+    Window    ctx_win;       /* None when closed */
+    int       ctx_app_idx;   /* which app row the ctx menu is for */
+    int       ctx_hov;       /* hovered item index, -1 = none */
+
     /* system tray (freedesktop.org XEmbed tray protocol) */
     Window    tray_icons[DB_TRAY_MAX];
     int       n_tray_icons;