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.

sw.js

@@ -11,7 +11,7 @@ let DYNAMIC_CACHE = 'securebit-pwa-dynamic-v4.7.56';
 // Build stamp — rewritten by scripts/post-build.js on every release so this file's
 // bytes change each deploy. That is what makes the browser detect a new Service Worker,
 // reinstall it, drop stale caches and (via controllerchange) prompt the page to update.
-const SW_BUILD_VERSION = '1786072827999';
+const SW_BUILD_VERSION = '1786294476636';
 
 // Load version from meta.json on install
 async function getAppVersion() {