refactor(ui): redesign with amber CRT terminal theme and mobile responsive

- Full visual overhaul: amber phosphor CRT aesthetic (gold on near-black)
- JetBrains Mono font family applied site-wide
- Terminal window style: LED amber dot on every panel header
- Header with 'user@shot-crafter:~$' title bar
- Login/register as terminal window cards
- Responsive: grid-5/grid-8 collapse on mobile, tables scroll horizontally
- Tabs with horizontal scroll on mobile
- Article headers wrap cleanly on small screens
- Custom scrollbars, selection color, focus states in amber theme
- New favicon in amber style
- term-section.html reusable partial created

src/main/resources/templates/PageResource/calculadora.html

@@ -1,10 +1,12 @@
 <!doctype html>
-<html lang="es" data-theme="light">
+<html lang="es">
 <head>
   <meta charset="UTF-8"/>
   <link rel="icon" type="image/svg+xml" href="/static/favicon.svg"/>
   <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
   <title>Calculadora — Calculadora de Craft de Shots</title>
+  <link rel="preconnect" href="https://fonts.googleapis.com">
+  <link rel="stylesheet" href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;500;700&display=swap">
   <link rel="stylesheet" href="/static/css/pico.min.css"/>
   <link rel="stylesheet" href="/static/css/app.css"/>
 </head>