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-lvgl/UITask.h

@@ -128,6 +128,7 @@ public:
   void setTrails(bool on);
   void setVectorMap(bool on);
   void simMapAt(const char* spec);      // Map tools > Live tiles
+  void simCompose(const char* text);    // the open thread's field: this text, keyboard up
   // Navigation map (NavMap.h)
   void navTargetsPopup();
   void navClosePopup();
@@ -169,6 +170,8 @@ public:
   void bannerOpen();   // the message banner tapped: its conversation
   void back();
   void sendFromCompose();
+  int composeLimit() const;                 // bytes the open thread's message may take
+  int composeLen(const char* typed) const;  // what `typed` takes once sent
   void setKeyboardVisible(bool show);
   void showToast(const char* text, uint32_t ms = 2500);
   // Clock tools (ClockScreen.h)