feat(repeaters): My Repeaters panel (stage 1)

New full-screen panel (main menu / FAB) for repeater administration:
- repeaters table (saved per-pubkey admin password, login metadata);
  plaintext per observer_brokers precedent (single-user LAN app)
- REST /api/repeaters: list merged with device contact truth, add
  (device REP contacts only), set/clear password, remove, login
- device_manager: _repeater_lock serializes all repeater ops (companion
  firmware has a single pending-request slot); repeater_login now
  filters LOGIN_SUCCESS by pubkey_prefix and captures is_admin/
  permissions into an in-memory session store
- login timeout UX: wrong password and unreachable repeater are
  indistinguishable (firmware stays silent) - error message names both
- panel UI: add-picker, password modal (remembered password), per-
  repeater path editor reusing /api/contacts/<pk>/paths + Leaflet
  repeater map picker (adapted from the DM panel)
- meshcore pin bumped to >=2.3.7 (send_login_sync era API, fixed
  LOGIN_SUCCESS/LOGIN_FAILED parsing; container already ships 2.3.7)

Stage 2 (management panel with Status/Telemetry/Neighbors/CLI/
Settings/Actions tools) follows after user review.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/static/css/style.css

@@ -1631,6 +1631,12 @@ main {
     color: white;
 }
 
+/* My Repeaters FAB button (REP green) */
+.fab-repeaters {
+    background: linear-gradient(135deg, #198754 0%, #146c43 100%);
+    color: white;
+}
+
 /* Filter bar overlay - slides down from top of chat area */
 .filter-bar {
     position: absolute;