Add receiver health UX, fix restart crash, add interface scan selector

Receiver management improvements for the admin Receivers tab:

- Per-receiver health: server now tracks each connection's state
  (connecting/connected/down), message count, and last-message time, exposed
  via new GET /api/admin/receiver-status and POST /api/admin/test-receivers.
- Receivers tab redesign: live "Active Receivers" panel with colored
  indicators (green=receiving, yellow=connected/no data, red=unreachable),
  a clear select -> SAVE & APPLY -> auto-test flow, and selectable scan
  results with a "USE SELECTED RECEIVERS" action.
- Network scan interface selector: scan a chosen interface/subnet instead of
  every local subnet. Oversized subnets (>/20, e.g. a docker /16) are disabled
  in the UI and rejected server-side, eliminating slow/hanging scans.

Critical fix — server crash on receiver restart:

- Receiver connection tasks were awaited inside main()'s top-level
  asyncio.gather. Restarting receivers cancels those tasks, and the resulting
  CancelledError propagated into the gather and killed the whole server
  process (every action after a restart then failed). Receiver tasks now run
  as independent background tasks; restart cancels and awaits them with
  return_exceptions=True so cancellation can never tear down the server.

Also cache-bust admin.css/admin.js/pixel-editor.js (?v=) so updated assets
load without a manual hard-refresh, and surface a clear message instead of a
silent "Loading…" when the status endpoint is unavailable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

admin/admin.css

@@ -592,6 +592,99 @@ textarea {
     font-size: 11px;
 }
 
+/* ===================================================================
+   Receivers tab — status indicators & selectable scan results
+   =================================================================== */
+.rx-section-label {
+    font-family: 'Press Start 2P', monospace;
+    font-size: 10px;
+    color: #fcd444;
+    margin: 22px 0 12px;
+    padding-bottom: 6px;
+    border-bottom: 1px solid rgba(92, 148, 252, 0.3);
+}
+
+.rx-summary {
+    font-family: 'Courier New', monospace;
+    font-size: 12px;
+    color: #b4d4ec;
+}
+
+#receiver-status-list {
+    display: flex;
+    flex-direction: column;
+    gap: 8px;
+    margin-bottom: 12px;
+}
+
+.rx-status-item {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    background: rgba(0, 0, 0, 0.3);
+    border: 1px solid rgba(92, 148, 252, 0.4);
+    border-radius: 6px;
+    padding: 8px 12px;
+    font-size: 13px;
+    flex-wrap: wrap;
+}
+
+.rx-dot {
+    width: 12px;
+    height: 12px;
+    border-radius: 50%;
+    flex-shrink: 0;
+    box-shadow: 0 0 6px currentColor;
+}
+.rx-dot.green  { background: #54fc54; color: #54fc54; }
+.rx-dot.yellow { background: #fcd444; color: #fcd444; }
+.rx-dot.red    { background: #fc5454; color: #fc5454; }
+.rx-dot.gray   { background: #888;    color: #888; box-shadow: none; }
+
+.rx-ip {
+    font-family: 'Courier New', monospace;
+    font-weight: bold;
+    color: #fcfcfc;
+    min-width: 110px;
+}
+
+.rx-label {
+    font-family: 'Press Start 2P', monospace;
+    font-size: 8px;
+}
+.rx-green  { color: #54fc54; }
+.rx-yellow { color: #fcd444; }
+.rx-red    { color: #fc5454; }
+.rx-gray   { color: #888; }
+
+.rx-detail {
+    color: #b4d4ec;
+    font-size: 11px;
+    margin-left: auto;
+}
+
+.rx-scan-item {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    padding: 6px 4px;
+    cursor: pointer;
+    border-bottom: 1px solid rgba(92, 148, 252, 0.2);
+}
+
+.rx-scan-item input[type="checkbox"] {
+    width: 16px;
+    height: 16px;
+    accent-color: #fcd444;
+    cursor: pointer;
+}
+
+.rx-scan-ip {
+    font-family: 'Courier New', monospace;
+    font-weight: bold;
+    color: #fcfcfc;
+}
+
 /* ===================================================================
    Pixel Editor
    =================================================================== */