feat(appearance): left stripe by default, with fill and strength as choices

The first version filled the whole row at 24%, and in a real log that meant
every row was painted: nearly every contact has SOME QSL state, so colour was
present everywhere and stopped being information — a striped background with
the data behind it.

The default is now a 3px stripe down the left edge. Same signal, nothing lost
to read it. A filled row is still offered, with a strength slider, for
operators who want the block — and the rule cards in Settings preview whichever
is chosen, so the decision is made by looking rather than by imagining.

Drawn as an inset shadow rather than a border: a border would shift the cells
three pixels on coloured rows only, and the columns would stop lining up.

Style and strength are normalised rather than rejected — a value out of range
is a slider that got away, not a reason to reset the operator's colours.

frontend/wailsjs/go/models.ts

@@ -3010,6 +3010,8 @@ export namespace main {
 	}
 	export class RowColorSettings {
 	    enabled: boolean;
+	    style: string;
+	    intensity: number;
 	    rules: RowColorRule[];
 	
 	    static createFrom(source: any = {}) {
@@ -3019,6 +3021,8 @@ export namespace main {
 	    constructor(source: any = {}) {
 	        if ('string' === typeof source) source = JSON.parse(source);
 	        this.enabled = source["enabled"];
+	        this.style = source["style"];
+	        this.intensity = source["intensity"];
 	        this.rules = this.convertValues(source["rules"], RowColorRule);
 	    }