✨ (pagetop): Añade puntos de corte a Flex/FlexItem

- `Breakpoint` gana variante `Xxxl` y métodos `name()`/`min_width()`/
  `resolved()`, para consultarse contra el tema activo.
- `Theme::breakpoint_entry()` sustituye a `breakpoint_min_width()`:
  devuelve un `BreakpointEntry` con nombre y ancho mínimo.
- Nuevo `Responsive<T>`, valor en cascada mobile-first por punto de
  corte.
- Ejemplo `examples/intro-responsive.rs` con los patrones de uso.

examples/locale/en-US/intro-responsive.ftl

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+responsive_slogan = Responsive behavior with Flex
+responsive_note = Resize your browser window to see how the boxes below react. The breakpoints used are the usual Bootstrap ones: sm from 576px, md from 768px, lg from 992px.
+
+responsive_block_title_activation = Flex activation at a breakpoint
+responsive_block_title_direction = Direction change
+responsive_block_title_grid = Column grid
+responsive_block_title_justify_align = Justify and align
+responsive_block_title_order = Visual order
+responsive_block_title_gap_grow = Spacing and growth
+
+responsive_title_activation = Stacked menu on mobile, in a row from md
+responsive_title_direction = Image and text stacked on mobile; in a row, image on the right, from md
+responsive_title_grid = From one column on mobile to three on desktop
+responsive_title_justify_align = Centered header on mobile, spread out from md
+responsive_title_order = Sidebar after the content on mobile, before it from lg
+responsive_title_gap_grow = Growing spacing and a search box that only grows from md
+
+responsive_box_nav_home = Home
+responsive_box_nav_products = Products
+responsive_box_nav_about = About
+responsive_box_nav_contact = Contact
+
+responsive_box_image = Image
+responsive_box_text = Text
+
+responsive_box_card = Card { $n }
+
+responsive_box_logo = Logo
+responsive_box_menu = Menu
+
+responsive_box_content = Content
+responsive_box_sidebar = Sidebar
+
+responsive_box_file = File
+responsive_box_edit = Edit
+responsive_box_search = Search

examples/locale/es-ES/intro-responsive.ftl

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+responsive_slogan = Comportamiento responsive con Flex
+responsive_note = Cambia el ancho de la ventana del navegador para ver cómo reaccionan las siguientes cajas. Los puntos de corte usados son los habituales de Bootstrap: sm a partir de 576px, md a partir de 768px, lg a partir de 992px.
+
+responsive_block_title_activation = Activación de Flex por punto de corte
+responsive_block_title_direction = Cambio de dirección
+responsive_block_title_grid = Rejilla de columnas
+responsive_block_title_justify_align = Justificación y alineación
+responsive_block_title_order = Orden visual
+responsive_block_title_gap_grow = Espaciado y crecimiento
+
+responsive_title_activation = Menú apilado en móvil, en fila a partir de md
+responsive_title_direction = Imagen y texto apilados en móvil; en fila, con la imagen a la derecha, a partir de md
+responsive_title_grid = De una columna en móvil a tres en escritorio
+responsive_title_justify_align = Cabecera centrada en móvil, distribuida a partir de md
+responsive_title_order = Barra lateral después del contenido en móvil, antes a partir de lg
+responsive_title_gap_grow = Espaciado creciente y buscador que sólo crece a partir de md
+
+responsive_box_nav_home = Inicio
+responsive_box_nav_products = Productos
+responsive_box_nav_about = Nosotros
+responsive_box_nav_contact = Contacto
+
+responsive_box_image = Imagen
+responsive_box_text = Texto
+
+responsive_box_card = Tarjeta { $n }
+
+responsive_box_logo = Logotipo
+responsive_box_menu = Menú
+
+responsive_box_content = Contenido
+responsive_box_sidebar = Barra lateral
+
+responsive_box_file = Archivo
+responsive_box_edit = Editar
+responsive_box_search = Buscador