Bootstrap NebulaOS repo and desktop shell UI

Initialize the NebulaOS monorepo layout with top-level documentation, ignore rules, placeholder service/package directories, compositor docs plus a baseline Sway config, and session startup/desktop entry files. Add the first desktop shell prototype under `shells/desktop/ui` using React + Vite, including themed surface components (desktop, top bar, launcher, preview), mock Nebula APIs and shell bridge state wiring, reusable UI controls/icons, and supporting styles/config needed for local development.

compositor/README.md

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+# Compositor
+
+NebulaOS uses Sway as its Wayland compositor. Configuration lives in `sway/nebula-sway.conf`.

compositor/sway/nebula-sway.conf

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+# NebulaOS Sway configuration
+
+include /etc/sway/config.d/*
+
+set $mod Mod4
+
+input type:keyboard {
+    xkb_layout us
+}
+
+output * bg #1a1a2e solid_color
+
+bindsym $mod+Return exec $term
+bindsym $mod+Shift+q kill
+bindsym $mod+d exec $menu
+
+exec_always nebula-session