[fix](trx-rs): update PKT and AM default bandwidths

PKT: default 25 kHz, max 50 kHz (was 3 kHz / 25 kHz).
AM:  default 9 kHz,  max 20 kHz (was 6 kHz / 15 kHz).

Applied consistently across the UI mode-defaults table
(trx-frontend-http), server initial-mode logic (trx-server), and the
SoapySDR DSP channel (trx-backend-soapysdr).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -2097,11 +2097,11 @@ const MODE_BW_DEFAULTS = {
   CWR:    [500,    50,    2_000,  50],
   LSB:    [2_700,  300,   6_000,  100],
   USB:    [2_700,  300,   6_000,  100],
-  AM:     [6_000,  500,   15_000, 500],
+  AM:     [9_000,  500,   20_000, 500],
   FM:     [12_500, 2_500, 25_000, 500],
   WFM:    [180_000, 50_000,300_000,5_000],
   DIG:    [3_000,  300,   6_000,  100],
-  PKT:    [3_000,  300,  25_000,  100],
+  PKT:    [25_000, 300,  50_000,  500],
 };
 const MODE_BW_FALLBACK = [3_000, 300, 500_000, 100];