fix(companion): flush pending lazy contacts write on shutdown/power-off

Contact updates (advert path cache, new/removed contacts, favourites) are
lazily debounced (dirty_contacts_expiry, 5s) to avoid wearing flash on
every mesh packet. CMD_REBOOT already flushed this before rebooting, but
UITask::shutdown() -- the low-battery auto-shutdown and long-press
power-off path -- didn't, so under continuous mesh traffic (which keeps
re-arming the debounce timer) a pending write could sit unflushed for a
whole session and be lost when one of those paths fired. Add
MyMesh::flushDirtyContacts() and call it from shutdown() alongside the
existing savePrefs()/saveRTCTime() flush.

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

examples/companion_radio/MyMesh.h

@@ -297,6 +297,14 @@ public:
 
   void savePrefs() { _store->savePrefs(_prefs, sensors.node_lat, sensors.node_lon); }
   void saveRTCTime() { _store->saveRTCTime(); }
+  // Contact updates (new adverts, path/lastmod changes) are lazily debounced
+  // (see dirty_contacts_expiry) to avoid wearing flash on every packet --
+  // under regular mesh traffic the timer keeps getting re-armed, so it can
+  // stay pending for the device's whole uptime. CMD_REBOOT already flushes
+  // this before rebooting; UITask::shutdown() (low-battery auto-shutdown,
+  // long-press power-off) needs the same flush or a whole session's worth
+  // of learned contacts can be lost.
+  void flushDirtyContacts() { if (dirty_contacts_expiry) { saveContacts(); dirty_contacts_expiry = 0; } }
   DataStore* getDataStore() const { return _store; }
   void applyApc();   // (re)initialise Adaptive Power Control from prefs
   // Adaptive Power Control is suppressed while repeating: a repeater wants full,

examples/companion_radio/ui-new/UITask.cpp

@@ -2177,6 +2177,7 @@ void UITask::shutdown(bool restart){
   // the_mesh.savePrefs() call sites already do without a dirty check.
   the_mesh.savePrefs();
   the_mesh.saveRTCTime();
+  the_mesh.flushDirtyContacts();
 
   // Auto-save the live GPS trail before power-off when the user enabled it
   // (Tools › Trail › Settings › Auto-save). This covers the low-battery