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.

app.rb

@@ -132,31 +132,26 @@ helpers do
   def print_epson_test_page
     receipt = ReceiptBuilder.new
     receipt.align(:center).bold.size(:big).text("TM-T20II TEST\n").size(:normal).bold(false)
-    receipt.align(:left).text(('-' * 48) + "\n")
 
+    receipt.align(:left).section_separator('TEXT STYLES')
     receipt.text("Normal text\n")
     receipt.bold.text("Bold text\n").bold(false)
     receipt.underline.text("Underlined text\n").underline(false)
     receipt.size(:wide).text("Double width\n").size(:normal)
     receipt.size(:high).text("Double height\n").size(:normal)
     receipt.size(:big).text("Double both\n").size(:normal)
-    receipt.text(('-' * 48) + "\n")
 
-    receipt.align(:center)
-    receipt.text("EAN-13 barcode:\n")
-    receipt.ean13('2012345678902')
-    receipt.feed(1)
+    receipt.section_separator('BARCODE')
+    receipt.align(:center).ean13('2012345678902')
 
-    receipt.text("QR code:\n")
-    receipt.qr('https://git.priesol.net/randy/home-gitops')
-    receipt.feed(1)
+    receipt.section_separator('QR CODE')
+    receipt.align(:center).qr('https://git.priesol.net/randy/home-gitops')
 
-    receipt.text("Raster image:\n")
-    receipt.image(64, 64, checkerboard_bitmap)
-    receipt.feed(1)
+    receipt.section_separator('IMAGE')
+    receipt.align(:center).image(64, 64, checkerboard_bitmap)
 
-    receipt.align(:left).text(('-' * 48) + "\n")
-    receipt.text("Printed: #{Time.now.strftime('%Y-%m-%d %H:%M:%S')}\n")
+    receipt.section_separator
+    receipt.align(:left).text("Printed: #{Time.now.strftime('%Y-%m-%d %H:%M:%S')}\n")
     receipt.feed(3).cut
     receipt.print!(PRINTER_DEVICE_EPSON)
   end