refactor(ui): apply code review fixes — bugs, risk, optimisations, cleanup

Fixes 7 bugs (timing UB on unsigned long, abs() no-op, XBM CRC PROGMEM
scan, CayenneLPP heap thrash ×2, UTF-8 reply prefix, lock-seq reset on
wake), 3 risk items (notif melody stop guard, bot trigger pre-lowercase,
lock-seq cleared on display wake), plus optimisations (fmtMsgAge/
getTextWidth caching, RTTTL builder consolidated to NodePrefs, HP_*
constants and homePageLabel table moved to NodePrefs, DataStore flat
lambda schema, translateUTF8Static factored out) and minor cleanup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/ui-new/KeyboardWidget.h

@@ -46,6 +46,8 @@ struct KeyboardWidget {
     addPlaceholder("{time}");
   }
 
+  void clearPlaceholders() { _ph_count = 0; }
+
   void addPlaceholder(const char* ph) {
     if (_ph_count < KB_PH_MAX) {
       strncpy(_ph_buf[_ph_count], ph, KB_PH_LEN - 1);