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.

shells/desktop/ui/README.md

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+# Nebula Desktop UI
+
+React UI for Nebula Desktop shell chrome. It does not host application or game windows. Those render as Wayland / XWayland surfaces. The native host in `shells/desktop/native` will load this UI in WebKitGTK.
+
+## Surfaces
+
+The same codebase can render each Wayland layer on its own:
+
+| URL | Renders |
+|-----|---------|
+| `/?surface=preview` (default) | Combined browser mockup |
+| `/?surface=desktop` | Background layer only |
+| `/?surface=topbar` | Top bar only |
+| `/?surface=launcher` | Launcher only |
+
+The native host chooses the origin. Surface selection is a relative query parameter, not a hardcoded port.
+
+## Develop
+
+```powershell
+npm install
+npm run dev
+```
+
+```powershell
+npm run lint
+npm run build
+```
+
+Oxlint is the linter for this project.