[FEATURE] Add FTX-1 support

[mm5agm]

Prompted by rickhall-007 in Discussion #70 — has an FTX-1 and asked about support.

CAT manuals now in docs/manuals/ (FTX-1_AM_ENG_2506-B.pdf, FTX-1_CAT_OM_ENG_2508-C.pdf). Compared against the FTX-1 CAT manual:

• Same 2-letter command family as the already-supported FTdx10/FT-710 (AB/AC/AG/.../VS/VX etc.), same FA/FB 9-digit-Hz frequency format, same ID; handshake pattern.
• Identification code is 0840 (vs FTdx10's own code) — YWC doesn't currently hard-validate this against the selected Radio Model, so no gate needs bypassing.
• Frequency range is wider than FTdx10: 30 kHz – 470 MHz (covers 6m/2m/70cm as well as HF), vs FTdx10's HF/6m-only range. Needs a proper per-model range check rather than reusing FTdx10's.
• Still to verify from the manual/real hardware: roofing filter CAT support (FTdx10 has none — automatic), and whether there's any rear-panel IF output for SDR panadapter use (FTdx10/FT-710 have none, hence the "no IF tap, antenna-port SDR risk" warning in Settings).

Asked Rick to try selecting "FTdx10" as the Radio Model with his real FTX-1 connected (safe since there's no ID-code gate) and report what works/breaks, as a real-world shortcut ahead of building a proper "FTX-1" model entry.

Next steps once Rick reports back:

1. Add "FTX-1" to the Radio Model dropdown (Pages/Settings.cshtml) with its own frequency range validation.
2. Confirm roofing-filter and IF-output behaviour, add the relevant Settings.cshtml sections.
3. Wire up ID0840 recognition wherever FTdx10/FT-710's ID06xx codes are referenced.