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.

appearance.go

@@ -23,8 +23,18 @@ type RowColorRule struct {
 
 // RowColorSettings is the whole appearance block.
 type RowColorSettings struct {
-	Enabled bool           `json:"enabled"`
-	Rules   []RowColorRule `json:"rules"`
+	Enabled bool `json:"enabled"`
+	// Style is how the colour is shown: "bar" paints a stripe down the left edge,
+	// "tint" washes the row, "both" does each.
+	//
+	// A bar is the default because a filled row is a poor signal in a log where
+	// nearly every contact has SOME QSL state: colour that is always present
+	// stops being information and becomes a striped background, with the data
+	// behind it. The stripe says the same thing and costs nothing to read.
+	Style string `json:"style"`
+	// Intensity is the tint strength in percent. Only used by "tint"/"both".
+	Intensity int            `json:"intensity"`
+	Rules     []RowColorRule `json:"rules"`
 }
 
 // The rule ids, in priority order. The frontend matches on these and holds the
@@ -56,7 +66,20 @@ func normRowColors(s RowColorSettings) RowColorSettings {
 	for _, r := range s.Rules {
 		byID[r.ID] = r
 	}
-	out := RowColorSettings{Enabled: s.Enabled}
+	out := RowColorSettings{Enabled: s.Enabled, Style: s.Style, Intensity: s.Intensity}
+	switch out.Style {
+	case "bar", "tint", "both":
+	default:
+		out.Style = "bar"
+	}
+	// Clamped rather than rejected: the value only shapes a colour-mix, and a
+	// number outside the range is a slider that got away, not a fault worth
+	// resetting the operator's whole choice for.
+	if out.Intensity < 5 {
+		out.Intensity = 12
+	} else if out.Intensity > 45 {
+		out.Intensity = 45
+	}
 	for _, id := range rowColorOrder {
 		r := byID[id]
 		r.ID = id