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.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "5.9.1",
+  "version": "5.9.2",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {
@@ -11,7 +11,7 @@
     "dev": "npm run build && python -m http.server 8000",
     "watch": "npx tailwindcss -i src/styles/tw-input.css -o assets/tailwind.css --watch",
     "serve": "npx http-server -p 8000",
-    "test": "node tests/sas-verification.test.mjs && node tests/verification-gate.test.mjs && node tests/inbound-frame-authentication.test.mjs && node tests/control-frame-authorization.test.mjs && node tests/security-level-shape.test.mjs && node tests/desktop-download-links.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/notification-ephemeral-privacy.test.mjs && node tests/key-derivation-compat.test.mjs && node tests/key-exchange-e2e.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/voice-auto-accept.test.mjs && node tests/legacy-offer-purge.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs && node tests/webrtc-sdp.test.mjs && node tests/webrtc-video.test.mjs && node tests/webrtc-adaptation.test.mjs && node tests/session-recovery.test.mjs && node tests/qr-zip-bomb.test.mjs && node tests/ice-gathering-patience.test.mjs && node tests/version-consistency.test.mjs && node tests/double-ratchet.test.mjs && node tests/ratchet-integration.test.mjs && node tests/descriptor-sbq2.test.mjs && node tests/sbq2-key-exchange.test.mjs && node tests/qr-scan-single-frame.test.mjs"
+    "test": "node tests/sas-verification.test.mjs && node tests/verification-gate.test.mjs && node tests/inbound-frame-authentication.test.mjs && node tests/control-frame-authorization.test.mjs && node tests/security-level-shape.test.mjs && node tests/desktop-download-links.test.mjs && node tests/file-transfer-consent.test.mjs && node tests/incoming-message-sanitization.test.mjs && node tests/outgoing-message-integrity.test.mjs && node tests/secure-chat-features.test.mjs && node tests/notification-meta-forwarding.test.mjs && node tests/notification-ephemeral-privacy.test.mjs && node tests/key-derivation-compat.test.mjs && node tests/key-exchange-e2e.test.mjs && node tests/file-type-allowlist.test.mjs && node tests/voice-auto-accept.test.mjs && node tests/legacy-offer-purge.test.mjs && node tests/webrtc-privacy-mode.test.mjs && node tests/indexeddb-metadata-encryption.test.mjs && node tests/disconnect-cleanup.test.mjs && node tests/timer-lifecycle.test.mjs && node tests/file-transfer-cleanup.test.mjs && node tests/file-transfer-ui-cleanup.test.mjs && node tests/file-transfer-callback-propagation.test.mjs && node tests/debug-window-hooks.test.mjs && node tests/inbound-message-rate-limit.test.mjs && node tests/file-transfer-chunk-rate-limit.test.mjs && node tests/ice-servers-validation.test.mjs && node tests/sessions-reducer.test.mjs && node tests/webrtc-sdp.test.mjs && node tests/webrtc-video.test.mjs && node tests/webrtc-adaptation.test.mjs && node tests/session-recovery.test.mjs && node tests/qr-zip-bomb.test.mjs && node tests/ice-gathering-patience.test.mjs && node tests/version-consistency.test.mjs && node tests/double-ratchet.test.mjs && node tests/ratchet-integration.test.mjs && node tests/descriptor-sbq2.test.mjs && node tests/sbq2-key-exchange.test.mjs && node tests/qr-scan-single-frame.test.mjs && node tests/mobile-chat-layout.test.mjs"
   },
   "keywords": [
     "p2p",