Add mode READMEs and move Bigscreen app

Add comprehensive READMEs for Bigscreen and Desktop modes, and relocate the Tauri frontend prototype into a new Bigscreen/ subdirectory. This moves package.json/package-lock, src, src-tauri, assets, views, styles and related files into Bigscreen/ to separate the controller-first shell from top-level docs. Update the root README.md to reframe the project as "NebulaOS", describe Bigscreen/Desktop modes, the vision, tech stack and development notes. This reorganizes the repo layout for clearer mode separation and documentation.

Bigscreen/src/views/home/home.html

@@ -0,0 +1,60 @@
+<section class="view home-view" data-view="home">
+  <header class="shell-topbar">
+    <div class="shell-topbar-content">
+      <p class="shell-brand">Nebula OS</p>
+      <div class="shell-status">
+        <span class="shell-avatar" aria-hidden="true"></span>
+        <p class="shell-time" data-clock>--:--</p>
+      </div>
+    </div>
+    <div class="shell-accent-line"></div>
+  </header>
+
+  <section class="home-hero">
+    <p class="muted">Dashboard</p>
+    <h1 class="view-title">Jump back in</h1>
+  </section>
+
+  <section class="tile-rail" data-focus-root data-home-rail>
+    <button class="focusable tile dashboard-tile tile-large" data-focusable="true" data-row="0" data-col="0" data-target="library">
+      <div class="tile-content">
+        <span class="tile-icon" aria-hidden="true">📚</span>
+        <div class="tile-text">
+          <p class="tile-label">Library</p>
+          <p class="tile-meta">Your games & apps</p>
+        </div>
+      </div>
+      <div class="tile-accent-bar"></div>
+    </button>
+    <button class="focusable tile dashboard-tile" data-focusable="true" data-row="0" data-col="1" data-target="browser">
+      <div class="tile-content">
+        <span class="tile-icon" aria-hidden="true">🌐</span>
+        <div class="tile-text">
+          <p class="tile-label">Browser</p>
+          <p class="tile-meta">Explore the web</p>
+        </div>
+      </div>
+      <div class="tile-accent-bar"></div>
+    </button>
+    <button class="focusable tile dashboard-tile" data-focusable="true" data-row="0" data-col="2" data-target="settings">
+      <div class="tile-content">
+        <span class="tile-icon" aria-hidden="true">⚙️</span>
+        <div class="tile-text">
+          <p class="tile-label">Settings</p>
+          <p class="tile-meta">System configuration</p>
+        </div>
+      </div>
+      <div class="tile-accent-bar"></div>
+    </button>
+    <button class="focusable tile dashboard-tile" data-focusable="true" data-row="0" data-col="3" data-target="power">
+      <div class="tile-content">
+        <span class="tile-icon" aria-hidden="true">⏻</span>
+        <div class="tile-text">
+          <p class="tile-label">Power</p>
+          <p class="tile-meta">Sleep, restart, shut down</p>
+        </div>
+      </div>
+      <div class="tile-accent-bar"></div>
+    </button>
+  </section>
+</section>