release: v4.9.0 — full redesign + reworked offline mode

Ground-up visual redesign across the entire surface (landing, connection
setup, chat header, security verification report, file transfer, PWA
install/update/offline dialogs).

Offline reworked: store-and-forward queue (send while offline → queued,
delivered on reconnect), WhatsApp-style per-message delivery status
(sending/sent/delivered/not-sent) via delivery receipts, offline buffering
for messages to an offline peer, and offline state no longer leaking into
the connection indicator. Resilient chunked file transfer with retransmission
and auto-save. README + screenshots added.

logo/securebit-mark.svg

@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg version="1.1" width="700" height="760" viewBox="276 240 700 760"
+     xmlns="http://www.w3.org/2000/svg">
+  <defs>
+    <!-- Brushed-metal silver for the upper "S" hook -->
+    <linearGradient id="sbSilver" x1="0" y1="0" x2="0.35" y2="1">
+      <stop offset="0" stop-color="#fdfdff"/>
+      <stop offset="0.20" stop-color="#e7e7ec"/>
+      <stop offset="0.46" stop-color="#c4c4cb"/>
+      <stop offset="0.72" stop-color="#a4a4ac"/>
+      <stop offset="1" stop-color="#86868d"/>
+    </linearGradient>
+    <!-- Glossy orange for the body, tail and dots -->
+    <linearGradient id="sbOrange" x1="0" y1="0" x2="0.25" y2="1">
+      <stop offset="0" stop-color="#ffb84d"/>
+      <stop offset="0.27" stop-color="#ff9a33"/>
+      <stop offset="0.58" stop-color="#fb7d16"/>
+      <stop offset="1" stop-color="#db5d04"/>
+    </linearGradient>
+  </defs>
+
+  <!-- Upper silver hook -->
+  <path fill="url(#sbSilver)" fill-rule="nonzero"
+        d="m 835.26446,352.56633 102.39051,-103.90366 -418.64101,1.00877 c 0,0 -171.69323,1.22309 -222.43455,167.96079 -52.34251,171.99925 77.67556,253.20215 77.67556,253.20215 0,0 35.54922,23.82856 79.77792,31.68982 15.73869,2.39372 79.16695,1.09532 79.16695,1.09532 54.47377,-10.08773 41.40629,-81.22528 -10.65516,-77.67557 C 492.06451,630.5166 372.5156,615.45079 386.86464,469.07968 415.02639,353.31661 520.52712,353.57511 520.52712,353.57511 Z"/>
+
+  <!-- Orange body + chat tail -->
+  <path fill="url(#sbOrange)" fill-rule="nonzero"
+        d="m 289.24744,881.29522 95.22696,-95.94027 369.13823,1.06997 C 873.15229,774.31964 863.51011,647.63259 863.51011,647.63259 846.7608,546.1216 749.51871,545.49427 749.51871,545.49427 l -232.01791,0.25219 c -37.80546,-8.91638 -37.85435,-49.4299 -37.85435,-49.4299 0,0 -1.56131,-38.07813 40.52401,-46.63785 l 260.62023,-0.7745 c 170.83788,24.60922 185.61432,187.63187 185.61432,187.63187 0,0 18.85523,117.07655 -90.63794,200.89054 l -0.62454,154.4184 -144.79052,-110.68137 z"/>
+
+  <!-- Two chat dots -->
+  <path fill="url(#sbOrange)"
+        d="m 658.38568,658.74237 a 27.462458,27.462458 0 0 1 -27.43073,27.46244 27.462458,27.462458 0 0 1 -27.49412,-27.39898 27.462458,27.462458 0 0 1 27.36719,-27.52575 27.462458,27.462458 0 0 1 27.55736,27.33536 z"/>
+  <path fill="url(#sbOrange)"
+        d="m 748.42871,659.07971 a 27.462458,27.462458 0 0 1 -27.43073,27.46244 27.462458,27.462458 0 0 1 -27.49412,-27.39898 27.462458,27.462458 0 0 1 27.36719,-27.52575 27.462458,27.462458 0 0 1 27.55736,27.33537 z"/>
+</svg>