[feat](trx-rs): remap decoder modes — remove DIG/PKT from SDR, wire decoders to standard modes

For SDR backends, DIG and PKT are removed from supported_modes and
replaced by USB and FM respectively. CAT backends (FT-817, FT-450D)
retain DIG/PKT as before.

Decoder mode allowances updated:
- APRS: FM | PKT (was PKT only)
- HF-APRS: USB | DIG (was DIG only)
- AIS: AIS | FM | PKT (was AIS only)
- VDES: VDES | FM (was VDES only)
- FT8/FT4/FT2/WSPR: USB | DIG (unchanged)
- CW: CW | CWR (unchanged)
- LRPT: FM (unchanged, mode-independent)

Frontend status text, bookmark decoder toggles, background-decode
fallbacks, and scheduler wiring updated to match.

https://claude.ai/code/session_01DCAaMH8RF5FNB2gRtVu4pY
Signed-off-by: Claude <noreply@anthropic.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/plugins/background-decode.js

@@ -102,7 +102,7 @@
     if (supported.length > 0) return supported;
     const mode = String(bookmark && bookmark.mode || "").trim().toUpperCase();
     if (mode === "AIS") return ["ais"];
-    if (mode === "PKT") return ["aprs"];
+    if (mode === "PKT" || mode === "FM") return ["aprs"];
     return supported;
   }