Stage 3a: wrap the app in 7.css's .window.glass.active frame

The app rendered a bare .title-bar with no .window around it, which is why the
Aero frame, glass and drop shadow were missing and the title-bar controls sat
permanently in their inactive colours: EVERY control-glyph rule in 7.css is
keyed on `.title-bar.active` / `.window.active .title-bar`, and nothing ever
set .active.

- app.html's root is now `.window.glass`, so it picks up the 6px radius, the
  1px frame, the `inset 0 0 0 1px #fff9` highlight, the drop shadow and the
  backdrop-filter glass layer.
- The content below the title bar moved into `.window-body`, 7.css's recessed
  inner panel (6px inset + its own border) — the thing that gives a Win7 window
  its depth.
- `.active` binds to a new `windowFocused` signal, seeded from
  document.hasFocus() and updated from window focus/blur. The webview receives
  those when the NATIVE window's focus changes, so no RPC is needed.

Harness renders the shell focused and unfocused: active shows the red close
button and crisp glyphs, inactive desaturates, matching the reference.

Native window transparency (for the drop shadow and rounded corners to fall
outside the window rather than being clipped by an opaque surface) is
deliberately NOT in this commit — it is a native flag with a real chance of
misbehaving on Linux, so it gets its own change and a real-app check.

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

ui/dev/harness.html

@@ -51,9 +51,9 @@
         </div>
       </figure>
       <figure class="cell" style="margin:0">
-        <figcaption>ours — bare .title-bar, no wrapper</figcaption>
+        <figcaption>ours — app.html shell (active above, inactive below)</figcaption>
         <div class="stage">
-          <div style="width: 320px">
+          <div class="window glass active" style="width: 320px">
             <div class="title-bar">
               <div class="title-bar-text">miniSCP</div>
               <div class="title-bar-controls">
@@ -62,9 +62,18 @@
                 <button aria-label="Close"></button>
               </div>
             </div>
-            <div style="background: var(--w7-surface); padding: 10px">
-              <p style="margin:0">No frame, no shadow, no glass.</p>
+            <div class="window-body"><p style="margin:0">Focused window.</p></div>
+          </div>
+          <div class="window glass" style="width: 320px; margin-top: 12px">
+            <div class="title-bar">
+              <div class="title-bar-text">miniSCP</div>
+              <div class="title-bar-controls">
+                <button aria-label="Minimize"></button>
+                <button aria-label="Maximize"></button>
+                <button aria-label="Close"></button>
+              </div>
             </div>
+            <div class="window-body"><p style="margin:0">Unfocused — chrome desaturates.</p></div>
           </div>
         </div>
       </figure>