feat: open a conversation on its first unread message, under a "New" line

Both UIs: a channel, DM or room with unread messages opens on the oldest of
them (MessageHistory::firstUnreadChannel / firstUnreadDm, own messages not
counted), a "New" line above it, so it reads down to the newest. Nothing
unread: the newest, as before.

L1: the first unread is selected and scrolled up top with the line in view;
the channel's unread count now counts down as you read towards the newest.
L2: the page holding it, scrolled to the line; an incoming message scrolls
down only when the list is already at its end. Sim hooks: sim_unread (L1),
sim_open("unread") (L2).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-lvgl/UITask.h

@@ -130,6 +130,7 @@ public:
   void simMapAt(const char* spec);      // Map tools > Live tiles
   void simCompose(const char* text);    // the open thread's field: this text, keyboard up
   void simDm();                         // the first chat contact's thread
+  void simUnread();                     // read and new messages to open on
   void simSend() { sendFromCompose(); } // the field, sent
   // Navigation map (NavMap.h)
   void navTargetsPopup();
@@ -425,6 +426,7 @@ private:
   void buildNode();
   void refreshNode();
   void buildThread();
+  void threadOpenAt(int first);
   void refreshThread();
   void refreshThreadAges();
   uint32_t threadSignature() const;
@@ -539,6 +541,9 @@ private:
   bool     _thread_is_channel = false;
   int      _thread_skip = 0;          // newest messages above the page shown (Older / Newer)
   bool     _thread_scroll_top = false;
+  int      _thread_new = -1;          // on opening: newest-first index of the first unread (-1: none)
+  uint32_t _thread_new_key = 0;       // and that message, marked with "New" while the thread is open
+  bool     _thread_new_on = false;
   int      loadThreadPage(int& total);
   uint8_t  _thread_channel = 0;
   uint8_t  _thread_key[PUB_KEY_SIZE] = {0};