✨ (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

@@ -0,0 +1,36 @@
+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