Opt-in channel-power squelch for AM audio RX (#3315)

* AM: add opt-in channel-power squelch for AM audio RX

Add an SDR++-style power squelch on the complex channel signal for the
Analog Audio RX app in AM modes (DSB/SSB/CW). The squelch measures the
mean channel power, converts it to dBFS, and mutes the demodulated audio
when the signal is below a user-set threshold. Being carrier-based it
keys on the AM carrier (e.g. airband/ATC) rather than on audio-band
noise, which the existing FM squelch could not do for AM.

A new 'SQ' field (0-99, 0 = off) is added to the AM options view and is
persisted per app via app_settings. Default is 0, so existing AM
behaviour is unchanged unless the user enables it. Threshold maps 1..99
to -80..-20 dBFS.

Touches: message (AMConfigureMessage.squelch_level), baseband_api
(AMConfig::apply), receiver_model (am_squelch_level get/set),
app_settings (persistence), proc_am_audio (DSP), analog_audio_app (UI).

* Refactor audio power calculation to use magnitude squared

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

firmware/application/app_settings.hpp

@@ -40,7 +40,7 @@
 // Bring in the string_view literal.
 using std::literals::operator""sv;
 
-#define COMMON_APP_SETTINGS_COUNT 19
+#define COMMON_APP_SETTINGS_COUNT 20
 
 /* Represents a named setting bound to a variable instance. */
 /* Using void* instead of std::variant, because variant is a pain to dispatch over. */
@@ -150,6 +150,7 @@ struct AppSettings {
     uint8_t wfm_config_index = 0;
     uint8_t wfmam_config_index = 0;
     uint8_t squelch = 80;
+    uint8_t am_squelch = 0;  // AM channel-power squelch threshold (0 = off)
     uint8_t volume;
     // NOTE: update COMMON_APP_SETTINGS_COUNT when adding to this
 };