Phase 1/2 complete: system tray (XEMBED) + Plymouth boot splash

System tray in skydeskbar (Phase 1 #4):
- Claims _NET_SYSTEM_TRAY_S<screen> selection on startup and
  broadcasts MANAGER so nm-applet and other tray clients connect
- Handles _NET_SYSTEM_TRAY_OPCODE dock requests: XReparentWindow
  icon into the deskbar, sends XEMBED_EMBEDDED_NOTIFY
- Tray section is zero-height when empty; expands to DB_TRAY_H=26px
  when first icon embeds, shrinking back when last icon exits
- DestroyNotify / ReparentNotify clean up the icon list and
  reposition remaining icons; SelectionClear hands off gracefully
- Icon positions are updated (db_tray_reposition) whenever the app
  list changes and shifts the tray section vertically

Plymouth boot splash (Phase 2 #10):
- 'skyos' script-module theme: dark navy (#111520) background,
  "SkyOS" logo in accent blue (#90BBEE), thin progress bar
- 1×1 pixel bar_bg.png / bar_fg.png scaled at runtime by the
  Plymouth script — no large PNGs committed to the repo
- logo.png generated at build time by ImageMagick (same tool used
  for the wallpaper); build continues if ImageMagick is absent
- mkinitcpio HOOKS: added 'plymouth' after 'udev'
- Kernel cmdline: added 'splash' for UEFI systemd-boot entry
- customize_airootfs.sh: plymouth-set-default-theme skyos

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deskbar/include/deskbar.h

@@ -25,7 +25,9 @@
  *  ├──────────────────────────┤
  *  │  [1]  [2]  [3]  [4]     │  <- workspace strip
  *  ├──────────────────────────┤
- *  │ CPU 12%    MEM 45%       │  <- status bar
+ *  │ [⊙][📶][🔋]             │  <- system tray (XEMBED; hidden when empty)
+ *  ├──────────────────────────┤
+ *  │ VOL 50%  CPU 12%  MEM 45%│  <- status bar
  *  └──────────────────────────┘
  */
 
@@ -37,6 +39,9 @@
 #define DB_ICON_SZ     18      /* app icon square size */
 #define DB_WS_H        28      /* workspace strip height */
 #define DB_STATUS_H    22      /* status bar height */
+#define DB_TRAY_H      26     /* tray section height when icons are present */
+#define DB_TRAY_ICON_SZ 20    /* embedded icon window size */
+#define DB_TRAY_MAX    16     /* maximum simultaneous tray icons */
 #define DB_MAX_APPS    32
 
 /* ── fly-out menu ────────────────────────────────────────────────────── */
@@ -133,6 +138,14 @@ typedef struct Deskbar {
     int       sub_active;   /* which sub-menu is open: 0=Apps, 1=Settings, -1=none */
     int       sub_hov;      /* hovered item in sub-menu */
     int       sw_cache;     /* cached screen width for menu positioning */
+
+    /* system tray (freedesktop.org XEmbed tray protocol) */
+    Window    tray_icons[DB_TRAY_MAX];
+    int       n_tray_icons;
+    Atom      atom_tray_s0;     /* _NET_SYSTEM_TRAY_S<screen> */
+    Atom      atom_tray_opcode; /* _NET_SYSTEM_TRAY_OPCODE */
+    Atom      atom_xembed;      /* _XEMBED */
+    Atom      atom_manager;     /* MANAGER */
 } Deskbar;
 
 void db_init(Deskbar *db);