feat(antenna): per-band tune frequency for Ultrabeam and SteppIR

The band buttons in Station Control tuned to a fixed mid-band frequency baked
into the frontend. An operator who lives in the CW segment, or in the FT8
window, got the antenna resonant somewhere he never operates and had to nudge
it every time. The SteppIR's own controller has a Frequency (KHz) column per
band for exactly this; this is that column.

Stored sparsely: a band with no entry uses its default, so nothing migrates
and an operator sets only the bands he cares about. Left empty the Settings
box shows the default as its placeholder, which makes clearing it the obvious
way back.

The value is checked against the band plan before it is kept, because it goes
to the antenna as a tune command — a lost digit (1450 for 20 m) or kHz typed
as MHz would send the elements travelling to a length wrong for the band the
operator is on, and on a SteppIR that journey inhibits transmit the whole way.
A rejected entry is logged and the band falls back to its default.

Resolution happens in the backend and rides on the existing status poll, so
the widget no longer decides where a band button goes and cannot drift from
what Settings shows. Its own table stays only as a floor for the first poll.

frontend/wailsjs/go/models.ts

@@ -3286,6 +3286,7 @@ export namespace main {
 	    track_mode: string;
 	    tx_inhibit: boolean;
 	    bands: string[];
+	    band_freqs: Record<string, number>;
 	    freq_min_mhz: number;
 	    freq_max_mhz: number;
 	
@@ -3307,6 +3308,7 @@ export namespace main {
 	        this.track_mode = source["track_mode"];
 	        this.tx_inhibit = source["tx_inhibit"];
 	        this.bands = source["bands"];
+	        this.band_freqs = source["band_freqs"];
 	        this.freq_min_mhz = source["freq_min_mhz"];
 	        this.freq_max_mhz = source["freq_max_mhz"];
 	    }
@@ -3324,6 +3326,7 @@ export namespace main {
 	    step_khz: number;
 	    track_mode: string;
 	    bands: string[];
+	    band_freqs: Record<string, number>;
 	
 	    static createFrom(source: any = {}) {
 	        return new UltrabeamStatusInfo(source);
@@ -3343,6 +3346,7 @@ export namespace main {
 	        this.step_khz = source["step_khz"];
 	        this.track_mode = source["track_mode"];
 	        this.bands = source["bands"];
+	        this.band_freqs = source["band_freqs"];
 	    }
 	}
 	export class UpdateInfo {