[feat](trx-rs): add FT4 decoder support

Reuse the existing ft8_lib C library (FTX_PROTOCOL_FT4) and FT8
decoder infrastructure to add FT4 decoding across the full stack.

Changes:
- trx-ft8: add protocol param to ft8_decoder_create; add Ft8Decoder::new_ft4()
- trx-core: DecodedMessage::Ft4 variant, AUDIO_MSG_FT4_DECODE (0x14),
  ft4_decode_enabled/ft4_decode_reset_seq state, SetFt4DecodeEnabled/
  ResetFt4Decoder commands, protocol mapping
- trx-server: DecoderHistories::ft4, run_ft4_decoder (7.5s slots via
  now*2/15), run_background_ft4_decoder, history push/replay, decoder
  task spawn
- trx-frontend-http: ft4_history in FrontendRuntimeContext,
  toggle/clear endpoints, /ft4.js route, bookmark/scheduler/background
  decode support, DecodeHistoryPayload ft4 field
- web: ft4.js plugin (7.5s period timer, reuses FT8 CSS/map infra),
  FT4 subtab in index.html, app.js dispatch (onServerFt4/Batch,
  restoreFt4History), decode-history-worker HISTORY_GROUP_KEYS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/decode-history-worker.js

@@ -1,5 +1,5 @@
 const textDecoder = typeof TextDecoder === "function" ? new TextDecoder() : null;
-const HISTORY_GROUP_KEYS = ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "wspr"];
+const HISTORY_GROUP_KEYS = ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "ft4", "wspr"];
 
 function decodeCborUint(view, bytes, state, additional) {
   const offset = state.offset;