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.cpp

@@ -2190,6 +2190,13 @@ void UITask::gotoMessagesScreen() {
   setCurrScreen(messages_screen);
 }
 
+#ifdef SIM_PLATFORM
+void UITask::simCompose(const char* text) {
+  gotoMessagesScreen();
+  ((MessagesScreen*)messages_screen)->simCompose(text);
+}
+#endif
+
 void UITask::openContactDM(const ContactInfo& ci) {
   ((MessagesScreen*)messages_screen)->reset();
   ((MessagesScreen*)messages_screen)->enterDM(ci);
@@ -2661,6 +2668,11 @@ extern "C" EMSCRIPTEN_KEEPALIVE void sim_enqueue_key(char c) {
   if (g_sim_ui_task_for_js) g_sim_ui_task_for_js->injectSimKey(c);
 }
 
+// Sim tests: a channel post's keyboard, holding `text`.
+extern "C" EMSCRIPTEN_KEEPALIVE void sim_compose(const char* text) {
+  if (g_sim_ui_task_for_js) g_sim_ui_task_for_js->simCompose(text);
+}
+
 // Long-press counterpart -- the host page's own press-and-hold timer (see
 // web/index.html/mesh.html) calls this instead of sim_enqueue_key() once a
 // button/key has been held past the same ~1000ms threshold every real