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-new/UITask.h

@@ -284,6 +284,7 @@ public:
 #ifdef SIM_PLATFORM
   void simCompose(const char* text);   // sim tests: a channel post's keyboard holding `text`
   void simMessage(const char* text);   // sim tests: a channel message arrives
+  void simUnread(int which);           // sim tests: read and new messages, opened
 #endif
   void openContactDM(const ContactInfo& ci);
   void openChannelHistory(uint8_t channel_idx);   // Favourites dial: open a pinned channel