feat(style): Mejorar derivePanelPalette con HSL

- Header usa darkenHsl/lightenHsl (preserva tono mejor que RGB)
- Selección sin foco usa desaturate + lightenHsl (más elegante)
- Nueva función contrastTextColor() para texto automático b/n
- Test adicional para contrastTextColor

Z-Design ahora produce paletas más armónicas desde color base.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/style.zig

@@ -1048,12 +1048,13 @@ fn deriveDarkPalette(base: Color) PanelColorScheme {
         .etiquetas = white.darken(30), // ~70% brightness
         .placeholder = gray,
 
-        // Header: darkened base color
+        // Header: darkened using HSL (preserves hue better than RGB darken)
-        .header = base.darken(60),
+        .header = base.darkenHsl(50),
 
-        // Selection: where the base color SHINES
+        // Selection: base color at full strength when focused
-        .seleccion_fondo_con_focus = base, // Full color!
+        .seleccion_fondo_con_focus = base,
-        .seleccion_fondo_sin_focus = base.blendTowards(gray, 50), // Muted
+        // Unfocused: desaturated and slightly lightened (HSL-based, more elegant)
+        .seleccion_fondo_sin_focus = base.desaturate(60).lightenHsl(10),
 
         // Borders: accent on focus, subtle otherwise
         .borde_con_focus = base,
@@ -1079,12 +1080,13 @@ fn deriveLightPalette(base: Color) PanelColorScheme {
         .etiquetas = black.lighten(40), // ~60% darkness
         .placeholder = gray,
 
-        // Header: very light version of base
+        // Header: lightened using HSL (preserves hue better)
-        .header = base.blendTowards(white, 85),
+        .header = base.lightenHsl(40),
 
-        // Selection: base color (slightly lightened for readability)
+        // Selection: base color at full strength when focused
         .seleccion_fondo_con_focus = base,
-        .seleccion_fondo_sin_focus = base.blendTowards(white, 60),
+        // Unfocused: desaturated and lightened (HSL-based)
+        .seleccion_fondo_sin_focus = base.desaturate(50).lightenHsl(30),
 
         // Borders: accent on focus, subtle otherwise
         .borde_con_focus = base,
@@ -1092,6 +1094,17 @@ fn deriveLightPalette(base: Color) PanelColorScheme {
     };
 }
 
+/// Get appropriate text color (black or white) based on background luminosity.
+/// Uses the HSL lightness value to determine contrast.
+pub fn contrastTextColor(background: Color) Color {
+    const hsl = background.toHsl();
+    // If background is light (L > 0.5), use dark text; otherwise use light text
+    return if (hsl.l > 0.5)
+        Color.rgb(20, 20, 25) // Dark text for light backgrounds
+    else
+        Color.rgb(245, 245, 245); // Light text for dark backgrounds
+}
+
 // =============================================================================
 // Z-Design Tests
 // =============================================================================
@@ -1149,3 +1162,20 @@ test "derivePanelPalette light mode" {
     try std.testing.expect(palette.fondo_con_focus.g > 245);
     try std.testing.expect(palette.fondo_con_focus.b > 250);
 }
+
+test "contrastTextColor" {
+    // Dark background should get light text
+    const dark_bg = Color.rgb(30, 30, 30);
+    const text_on_dark = contrastTextColor(dark_bg);
+    try std.testing.expect(text_on_dark.r > 200); // Light text
+
+    // Light background should get dark text
+    const light_bg = Color.rgb(240, 240, 240);
+    const text_on_light = contrastTextColor(light_bg);
+    try std.testing.expect(text_on_light.r < 50); // Dark text
+
+    // Mid-gray (128/255 = 0.502 > 0.5) gets dark text
+    const mid_gray = Color.rgb(128, 128, 128);
+    const text_on_mid = contrastTextColor(mid_gray);
+    try std.testing.expect(text_on_mid.r < 50); // Dark text (L = 0.502 > 0.5)
+}