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.

doc/CRYPTOGRAPHY.md

@@ -7,7 +7,7 @@ this document describes.
 
 | | |
 | --- | --- |
-| Release | v5.9.1 |
+| Release | v5.9.2 |
 | Protocol version | 4.1 |
 | Ratchet wire version | 1 |
 
@@ -238,5 +238,5 @@ worse than one that reports nothing.
 
 ## Scope
 
-This describes the browser implementation as it stands in v5.9.1. It is not a
+This describes the browser implementation as it stands in v5.9.2. It is not a
 substitute for independent cryptographic review.