fix(mobile): make the chat layout responsive on phones; release v5.9.2

The chat did not lay out correctly on phones, iPhone worst of all: the header
would not stay at the top, and the layout shifted as you scrolled or opened the
keyboard.

The root cause was a floor, not a position. The app shell and column both carry
.minimal-bg, which sets min-height:100vh — and min-height always beats height. On
iOS 100vh is the URL-bar-retracted *large* viewport, so with the bar showing the
shell was held some 60-100px taller than the visible area. That surplus made the
document scrollable, and a scrolling document is how the header rode off the top.
It also left the visual-viewport tracking inert: forcing --sb-vh to 700px left the
shell at 844px. With the floor removed it follows to 700px and the document does
not scroll.

Also: the app shell is now sized from visualViewport rather than pinned, the
message list is the only scroller and can actually shrink, safe-area padding
applies (viewport-fit=cover) and collapses under the keyboard, and the
online/offline toast no longer sits on top of the header.

Adds ?preview=chat, which renders the chat layout with canned content and no
connection — presentational only, no peer manager, keys, or network.

Verified in an emulated iPhone viewport at rest and with the keyboard open:
header pinned at 0-64, composer flush to the keyboard line, document unscrollable.

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1786072827999",
-  "buildVersion": "1786072827999",
-  "appVersion": "5.9.1",
-  "buildTime": "2026-08-07T03:20:28.043Z",
-  "buildId": "1786072827999-808fd99",
-  "gitHash": "808fd99",
+  "version": "1786294476636",
+  "buildVersion": "1786294476636",
+  "appVersion": "5.9.2",
+  "buildTime": "2026-08-09T16:54:36.685Z",
+  "buildId": "1786294476636-556727e",
+  "gitHash": "556727e",
   "generated": true,
-  "generatedAt": "2026-08-07T03:20:28.044Z"
+  "generatedAt": "2026-08-09T16:54:36.686Z"
 }