feat: a message's bytes left, counted as it will be sent, on every screen

The limit is what the send leaves of MAX_TEXT_LEN: a channel post carries
"<node name>: ", a direct message resent more than three times keeps two
bytes spare, and placeholders count as they expand (msgtext::limit/sentLen).
Bytes, not characters. The path and packet header sit outside the payload;
static_asserts keep a full message within the radio frame on the longest path.

From 40 bytes left the count shows: on the L2 as a tag on the field's top
edge, on OLED/e-ink at the right of the preview's last line with the text
wrapped short of it. Input stops at the limit; a send still over (a
placeholder grew) is held. Sim hooks to open a compose with text.

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

examples/companion_radio/ui-new/UITask.h

@@ -274,6 +274,9 @@ public:
   void gotoHomeScreen() { setCurrScreen(home); }
   void gotoSettingsScreen();
   void gotoMessagesScreen();
+#ifdef SIM_PLATFORM
+  void simCompose(const char* text);   // sim tests: a channel post's keyboard holding `text`
+#endif
   void openContactDM(const ContactInfo& ci);
   void openChannelHistory(uint8_t channel_idx);   // Favourites dial: open a pinned channel
   void openRoomServer(const ContactInfo& ci);     // Favourites dial: open a pinned room (logs in first)