feat: agregar selector de tema oscuro/claro

Toggle sol/luna en el navbar que permite cambiar entre tema oscuro (por
defecto) y tema claro. Preferencia guardada en localStorage. Incluye
overrides de CSS para todos los componentes con colores hardcodeados,
inversión de logo, y transición suave entre temas.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

assets/js/main.js

@@ -177,4 +177,39 @@ document.addEventListener('DOMContentLoaded', function () {
         }
     });
 
+    // ================================================================
+    // 7. Dark / Light Theme Toggle
+    // ================================================================
+    var themeToggle = document.getElementById('themeToggle');
+    var themeIcon   = document.getElementById('themeIcon');
+
+    function setTheme(theme) {
+        document.documentElement.setAttribute('data-theme', theme);
+        localStorage.setItem('theme', theme);
+        if (themeIcon) {
+            themeIcon.className = theme === 'light' ? 'fas fa-sun' : 'fas fa-moon';
+        }
+        // Update theme-color meta tag
+        var metaTheme = document.querySelector('meta[name="theme-color"]');
+        if (metaTheme) {
+            metaTheme.setAttribute('content', theme === 'light' ? '#f5f5f5' : '#0a0a0a');
+        }
+    }
+
+    // Determine initial theme: localStorage > system preference > dark
+    var savedTheme = localStorage.getItem('theme');
+    if (savedTheme) {
+        setTheme(savedTheme);
+    } else if (window.matchMedia && window.matchMedia('(prefers-color-scheme: light)').matches) {
+        setTheme('light');
+    }
+    // else: default is dark (no data-theme attribute needed)
+
+    if (themeToggle) {
+        themeToggle.addEventListener('click', function () {
+            var current = document.documentElement.getAttribute('data-theme');
+            setTheme(current === 'light' ? 'dark' : 'light');
+        });
+    }
+
 });