Feat v0.6.10 viewport foundation

Single layout model: body → shell → surface. CSS zoom replaces
transform: scale() — no more getBoundingClientRect hacks. Extension
surfaces use height: 100% instead of 100vh. All viewport-height
declarations use 100dvh fallbacks for mobile browsers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/chat/css/main.css

@@ -12,7 +12,7 @@
 .chat-app {
   display: flex;
   flex-direction: column;
-  height: 100vh;
+  height: 100%;
   overflow: hidden;
   background: var(--bg);
   color: var(--text);
@@ -22,7 +22,7 @@
   display: flex;
   align-items: center;
   justify-content: center;
-  height: 100vh;
+  height: 100%;
 }
 
 .chat-body {