FLEX pager app (#2883)

* Add FLEX pager support

- Introduced a new FLEX configuration function in baseband_api.
- Added FLEX application view and associated UI elements.
- Implemented FLEX processing logic in proc_flex, including demodulation and message handling.
- Updated CMakeLists to include new FLEX source files and headers.
- Enhanced message system to support FLEX-specific messages and statistics.

This commit lays the groundwork for FLEX pager functionality, allowing for the reception and processing of FLEX messages.

* Fixed baseband and moved app to external with some other fixes.

* Format code

---------

Co-authored-by: RocketGod <57732082+RocketGod-git@users.noreply.github.com>

firmware/application/external/flex_rx/ui_flex_rx.cpp

@@ -0,0 +1,74 @@
+#include "ui_flex_rx.hpp"
+
+#include "baseband_api.hpp"
+#include "portapack_persistent_memory.hpp"
+#include "string_format.hpp"
+#include "memory_map.hpp"
+#include "usb_serial_asyncmsg.hpp"
+
+using namespace portapack;
+
+namespace ui::external_app::flex_rx {
+
+FlexAppView::FlexAppView(NavigationView& nav)
+    : nav_{nav} {
+    baseband::run_prepared_image(portapack::memory::map::m4_code.base());
+
+    add_children({&field_frequency,
+                  &field_rf_amp,
+                  &field_lna,
+                  &field_vga,
+                  &rssi,
+                  &console});
+
+    field_frequency.set_value(receiver_model.target_frequency());
+    field_frequency.updated = [this](rf::Frequency f) {
+        update_freq(f);
+    };
+
+    receiver_model.set_sampling_rate(3072000);
+    receiver_model.set_baseband_bandwidth(1750000);
+    receiver_model.enable();
+    receiver_model.set_squelch_level(0);
+
+    baseband::set_flex_config();
+}
+
+FlexAppView::~FlexAppView() {
+    receiver_model.disable();
+    baseband::shutdown();
+}
+
+void FlexAppView::focus() {
+    field_frequency.focus();
+}
+
+void FlexAppView::update_freq(rf::Frequency f) {
+    receiver_model.set_target_frequency(f);
+}
+
+void FlexAppView::on_packet(const FlexPacketMessage* message) {
+    std::string text = "";
+
+    text += "FLEX ";
+    text += to_string_dec_uint(message->packet.bitrate);
+    text += " ";
+    text += to_string_dec_uint(message->packet.capcode);
+    text += ": ";
+    text += message->packet.message;
+
+    console.writeln(text);
+}
+
+void FlexAppView::on_stats(const FlexStatsMessage* /* message */) {
+}
+
+void FlexAppView::on_debug(const FlexDebugMessage* message) {
+    std::string text = "DBG: ";
+    text += message->text;
+    text += " " + to_string_hex(message->val1, 8);
+    text += " " + to_string_hex(message->val2, 8);
+    console.writeln(text);
+}
+
+}  // namespace ui::external_app::flex_rx