feat(ui): open DM from filled Favourites tile

Enter on a populated slot of the Favourites grid now jumps straight into
that contact's DM history — resolves the pinned 6-byte pub_key prefix
against the contact list, then routes through new
UITask::openContactDM(ci) which resets QuickMsgScreen and calls
enterDM() to land directly in DM_HIST.

Empty slots still show the "pin from contact options" hint until phase 3
wires the in-grid mini-picker.

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

examples/companion_radio/ui-new/UITask.h

@@ -108,6 +108,7 @@ public:
   void gotoHomeScreen() { setCurrScreen(home); }
   void gotoSettingsScreen();
   void gotoQuickMsgScreen();
+  void openContactDM(const ContactInfo& ci);
   void gotoToolsScreen();
   void gotoRingtoneEditor(int slot = 0);
   void gotoBotScreen();