[feat](trx-rs): add FT2 decoder support (wired to FT4)

Mirrors the FT4 implementation across the full stack. The trx-ft8
crate wires Ft8Decoder::new_ft2() to FTX_PROTOCOL_FT4 as a
placeholder pending a dedicated FT2 implementation.

Changes:
- trx-ft8: Ft8Decoder::new_ft2() delegates to with_protocol(Ft4)
- trx-core: DecodedMessage::Ft2, AUDIO_MSG_FT2_DECODE (0x15),
  ft2_decode_enabled/ft2_decode_reset_seq state, SetFt2DecodeEnabled/
  ResetFt2Decoder commands, protocol mapping
- trx-server: DecoderHistories::ft2, run_ft2_decoder (7.5s slots),
  run_background_ft2_decoder, history push/replay, decoder task spawn
- trx-frontend-http: ft2_history in FrontendRuntimeContext,
  toggle/clear endpoints, /ft2.js route, bookmark/scheduler/background
  decode support, DecodeHistoryPayload ft2 field
- web: ft2.js plugin (3.75s period timer), FT2 subtab in index.html,
  FT2 map source (distinct hue), app.js dispatch, decode-history-worker
  HISTORY_GROUP_KEYS, bookmarks read/write/apply

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", "ft4", "wspr"];
+const HISTORY_GROUP_KEYS = ["ais", "vdes", "aprs", "hf_aprs", "cw", "ft8", "ft4", "ft2", "wspr"];
 
 function decodeCborUint(view, bytes, state, additional) {
   const offset = state.offset;