Add visual separators: inverted banner sections on receipts, dividers in navbar

ReceiptBuilder#invert (GS B) + #section_separator give the test page (and
future receipts) a full-width white-on-black banner between sections --
the closest a direct-thermal printer gets to a second color. Navbar gets
a vr divider between Home | Brother/Shipping labels/Receipts | Labels to
group the three logical clusters.

lib/receipt_builder.rb

@@ -65,6 +65,19 @@ class ReceiptBuilder
     self
   end
 
+  # White-on-black reverse video (GS B n) -- the closest this direct-thermal
+  # printer gets to a second "color" for a separator/banner.
+  def invert(enable = true)
+    @buffer << (enable ? "\x1D\x42\x01" : "\x1D\x42\x00")
+    self
+  end
+
+  # A full-width inverted banner line, e.g. section_separator("SECTION").
+  def section_separator(label = '', width: 48)
+    padded = label.empty? ? (' ' * width) : " #{label} ".center(width)
+    feed(1).invert.text(padded[0, width]).invert(false).feed(1)
+  end
+
   def size(mode)
     @buffer << case mode
                when :big  then TXT_BIG