fix(brand/ux): real brand C on header, yellow read-tick, sign-in spinner, notif DP

- #5: use the ACTUAL brand C (extracted transparent white C + gold dot from the master
  app-icon) on the blue headers instead of a hand-drawn mark; login card keeps the app icon.
- #3: read-receipt double tick on my own (blue) bubble is now brand yellow (was blue-on-blue,
  invisible).
- #2: Sign-in button shows a spinner + 'Signing in…' on submit.
- #1: web notification uses the sender/group DP URL directly as the icon (drawing an external
  DP to canvas tainted it → silently fell back to initials).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/public/connect.html

@@ -76,7 +76,7 @@
 <script>if(new URLSearchParams(location.search).get('embed')==='1')document.documentElement.classList.add('embed');</script>
 <a href="/home" id="homeLink"><span data-ic="arrowLeft" data-sz="16"></span> Home</a>
 <div class="topbar" id="topbar">
-  <div class="brandrow"><img src="/mark-light.svg" alt="" style="height:46px;width:auto;max-width:190px;border-radius:8px;object-fit:contain;image-rendering:-webkit-optimize-contrast" onerror="this.replaceWith(Object.assign(document.createElement('div'),{className:'logo',textContent:'B'}))"><div class="brand">Biz <span>Connect</span></div></div>
+  <div class="brandrow"><img src="/mark-light.png" alt="" style="height:46px;width:auto;max-width:190px;border-radius:8px;object-fit:contain;image-rendering:-webkit-optimize-contrast" onerror="this.replaceWith(Object.assign(document.createElement('div'),{className:'logo',textContent:'B'}))"><div class="brand">Biz <span>Connect</span></div></div>
   <div class="agentchip" id="agentChip"></div>
 </div>
 <div class="topbar2" id="bar"><span id="barStatus"></span><button id="endBtn">End session</button></div>