feat: ADIF export field picker — choose exactly which fields to write

Global "Export to ADIF" gains a third option "Choose fields…" next to the
standard-only and full-OpsLog choices, and right-clicking selected QSOs adds
"Export selected — choose fields…". The picker separates official ADIF 3.1.7
fields (grouped by category) from OpsLog/non-standard tags actually present in
the log (discovered via DistinctExtraKeys over extras_json), with All/None per
group and reset-to-defaults; the selection is persisted per user.

Backend: adif.Exporter gains a Fields allow-set that gates every promoted and
extras field in writeRecord; app.go ExportADIF/ExportADIFFiltered/
ExportADIFSelected take a fields []string param, plus new ADIFExtraFields.

frontend/wailsjs/go/main/App.d.ts

@@ -28,6 +28,8 @@ export function ACOMSetOperate(arg1:boolean):Promise<void>;
 
 export function ACOMSetPower(arg1:boolean):Promise<void>;
 
+export function ADIFExtraFields():Promise<Array<string>>;
+
 export function ADIFFields():Promise<Array<adif.FieldDef>>;
 
 export function ADIFVersion():Promise<string>;
@@ -176,11 +178,11 @@ export function DuplicateProfile(arg1:number,arg2:string):Promise<profile.Profil
 
 export function ExplainAward(arg1:string,arg2:string):Promise<Array<main.AwardExplain>>;
 
-export function ExportADIF(arg1:string,arg2:boolean):Promise<adif.ExportResult>;
+export function ExportADIF(arg1:string,arg2:boolean,arg3:Array<string>):Promise<adif.ExportResult>;
 
-export function ExportADIFFiltered(arg1:string,arg2:boolean,arg3:qso.QueryFilter):Promise<adif.ExportResult>;
+export function ExportADIFFiltered(arg1:string,arg2:boolean,arg3:qso.QueryFilter,arg4:Array<string>):Promise<adif.ExportResult>;
 
-export function ExportADIFSelected(arg1:string,arg2:boolean,arg3:Array<number>):Promise<adif.ExportResult>;
+export function ExportADIFSelected(arg1:string,arg2:boolean,arg3:Array<number>,arg4:Array<string>):Promise<adif.ExportResult>;
 
 export function ExportAward(arg1:string):Promise<string>;