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/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