FLEX RX: fix long-address numeric decode, improve short message format (#3136)

- Fix long-address numeric message decoding: read body[0] from Vy (j+1)
  for multi-word messages, from w1 for single-word. Fix MF word count
  for long addresses (n_field words, not n_field+1).
- Fix long-address short message: read 5 additional BCD digits from Vy
  for 8-digit decode.
- Fix BCD table: index 10 is '.' (dot) not ' ' (space) in both RX and TX.
- Clean up address decode: identify address type by value range per
  Table 3.8.1-1. Remove is_group/is_temp_group from packet structs.
- Rename SMSG to SHORT. Format message payload directly in baseband:
  TONE, NUM abc, SRC N, SRC N N=M R=R, RESERVED hex.
- Add BIW1 heartbeat packet (biw_field=0xFF) on every decoded frame.
- Move timezone table to file scope, simplify console log_message,
  add No signal initial status, simplify serial BIW output format.

firmware/application/external/flex_rx/ui_flex_rx.hpp

@@ -37,13 +37,8 @@ class FlexAppView : public View {
     uint16_t status_cz_{0};
     uint16_t status_cc_{0};
 
-    // Message storage for console redraw
-    static constexpr size_t MAX_MESSAGES = 20;
-    std::vector<std::string> messages{};
-
     // Helper methods
     void log_message(const std::string& message);
-    void redraw_console();
     void update_freq(rf::Frequency f);
 
     // UI Elements - Row 0, dynamically positioned