fix(ui): don't nuke the registered panel's children on status update

Regression from 20375ec: the `signal-event reconnecting` and
`signal-event registered` handlers were assigning to
`directRegistered.textContent`, which is the PARENT element that
holds the entire registered UI — the "Registered — waiting"
header, incoming-call panel, recent-contacts section, call
history, the fingerprint-input bar, and the Call button. Setting
textContent on that parent wiped every child with a single text
node, so after registration the user saw "✅ Registered" with
NOTHING below it — no call input, no history, no call button.
App unusable post-registration.

Fix:
- Add a dedicated `#registered-status` <p> inside the header of
  `#direct-registered` (this element already existed as a plain
  paragraph without an id; just giving it an id).
- Rewrite both handlers to target that element by id instead of
  the parent, so `textContent =` only touches the status line
  and leaves the rest of the panel intact.
- The `registered` handler now also explicitly
  `registerBtn.classList.add("hidden")` and
  `directRegistered.classList.remove("hidden")` so the first
  register event correctly reveals the UI. Belt-and-braces for
  the transparent-reconnect case too — if the supervisor
  re-registers after a drop, the UI stays in the registered
  state.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/index.html

@@ -52,7 +52,7 @@
             <button id="register-btn" class="primary" style="background:#2196F3">Register on Relay</button>
             <div id="direct-registered" class="hidden" style="margin-top:12px">
               <div class="direct-registered-header">
-                <p style="color:var(--green);font-size:13px;margin:0">&#x2705; Registered — waiting for calls</p>
+                <p id="registered-status" style="color:var(--green);font-size:13px;margin:0">&#x2705; Registered — waiting for calls</p>
                 <button id="deregister-btn" class="secondary-btn small">Deregister</button>
               </div>
               <div id="incoming-call-panel" class="hidden" style="background:#1B5E20;padding:12px;border-radius:8px;margin:8px 0">