webui: add the Sources & Config sidebar section

Renames the "Add Source" section to "Sources & Config" and replaces the
fire-and-forget "Save list" button with Save and Reload, plus a one-line
status area that renders the hub's configSaved/configReloaded ack.

onConfigAck replaces the no-op stub Task 7 left behind. It branches on the
frame type because configSaved carries a path and configReloaded does not,
and surfaces the hub's error text on failure rather than failing silently.
The status is kept outside the DOM because buildSidebar() recreates this
section on every sources broadcast.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Client/udpstreamer/static/style.css

@@ -478,6 +478,16 @@ input[type=range].trig-range::-webkit-slider-thumb {
 .add-src-btn:hover { background:rgba(137,180,250,0.15); border-color:var(--accent); }
 .save-src-btn { color:var(--green); }
 .save-src-btn:hover { background:rgba(166,227,161,0.1); border-color:var(--green); }
+.cfg-btn-row { display:flex; gap:6px; }
+.cfg-btn-row .add-src-btn { flex:1; }
+.reload-src-btn { color:var(--peach); }
+.reload-src-btn:hover { background:rgba(250,179,135,0.1); border-color:var(--peach); }
+.cfg-status {
+  font-size:10px; line-height:1.3; padding:2px 0; min-height:13px;
+  overflow-wrap:anywhere;
+}
+.cfg-status.ok { color:var(--green); }
+.cfg-status.err { color:var(--red); }
 
 /* ── Stats panel ─────────────────────────────────────────────── */
 #stats-panel {