Unified /labels page (badges, edit, preview), mnemonic ESC/POS parser for the raw receipt field

- Saved labels/receipts now show on one page instead of split across
  Shipping labels and Receipts, each with a badge for its type/printer,
  Edit (name+content, in place), Print, and Delete.
- ZPL labels get a real visual preview via Labelary's public render API;
  ESC/POS ones show their raw content (no good general preview for that).
- The Receipts free-text field only ever encoded whatever was typed as
  literal printable characters -- pasting mnemonic ESC/POS notation
  (ESC/GS/LF as command names, 0xNN as raw bytes) just printed as text
  instead of being interpreted. Added a small line-based parser for that
  notation (ReceiptBuilder#raw appends already-resolved bytes verbatim),
  verified byte-for-byte against an example with align/bold/barcode/cut.

app.rb

@@ -107,16 +107,64 @@ helpers do
     receipt.print!(PRINTER_DEVICE_EPSON)
   end
 
-  # Prints already-formatted ESC/POS text as-is (just the reset + code page
-  # prefix and a trailing cut) -- for content prepared elsewhere or reprinted
-  # from the label repository, unlike print_receipt's title/body template.
+  ESCPOS_MNEMONICS = {
+    'ESC' => "\x1B", 'GS' => "\x1D", 'FS' => "\x1C", 'DLE' => "\x10",
+    'LF' => "\x0A", 'CR' => "\x0D", 'HT' => "\x09", 'NUL' => "\x00"
+  }.freeze
+
+  # Parses a line-based mnemonic ESC/POS notation, e.g.:
+  #   ESC a 0x01     # Align: Center
+  #   Hello          # literal text
+  #   LF
+  #   GS k 0x03 12345678 0x00
+  # Each `#...` trailing comment is stripped, then each line's
+  # whitespace-separated tokens become either a known control mnemonic, a
+  # raw `0xNN` byte, or literal text (CP852-encoded) -- consecutive literal
+  # tokens are rejoined with a single space so plain prose still reads
+  # correctly, while whitespace around control/byte tokens is dropped since
+  # those are just for human readability, not printer output.
+  def parse_escpos_notation(text)
+    bytes = "".b
+    text.each_line do |line|
+      prev_was_text = false
+      line.sub(/#.*/, '').split.each do |token|
+        if ESCPOS_MNEMONICS.key?(token)
+          bytes << ESCPOS_MNEMONICS[token]
+          prev_was_text = false
+        elsif token =~ /\A0x([0-9A-Fa-f]{2})\z/
+          bytes << [Regexp.last_match(1)].pack('H2')
+          prev_was_text = false
+        else
+          bytes << ' ' if prev_was_text
+          bytes << token.encode('CP852', invalid: :replace, undef: :replace, replace: '?')
+          prev_was_text = true
+        end
+      end
+    end
+    bytes
+  end
+
+  # Prints already-formatted ESC/POS notation (see parse_escpos_notation) --
+  # for content prepared elsewhere or reprinted from the label repository,
+  # unlike print_receipt's title/body template.
   def print_raw_escpos(content)
     receipt = ReceiptBuilder.new
-    receipt.align(:left).text(content)
+    receipt.align(:left).raw(parse_escpos_notation(content))
     receipt.feed(3).cut
     receipt.print!(PRINTER_DEVICE_EPSON)
   end
 
+  # Renders ZPL to a PNG via Labelary's public API for an inline preview.
+  # Fixed at the Datamax label size (4x6in @ 8dpmm ~= 203dpi) -- good enough
+  # for a thumbnail, not meant to be exact for other sizes.
+  def render_zpl_preview(zpl)
+    uri = URI('https://api.labelary.com/v1/printers/8dpmm/labels/4x6/0/')
+    res = Net::HTTP.post(uri, zpl, 'Content-Type' => 'application/x-www-form-urlencoded; charset=utf-8')
+    return nil unless res.is_a?(Net::HTTPSuccess)
+
+    res.body
+  end
+
   # Transparently reverse-proxies the current request to one of the
   # brother_ql_web instances, so all three printers (web UI and API alike)
   # are reachable through this single sinatra port instead of their own
@@ -166,7 +214,6 @@ get '/' do
 end
 
 get '/upload' do
-  @labels = LABEL_REPO.all(type: 'zpl')
   erb :upload
 end
 
@@ -234,7 +281,6 @@ post '/test-print' do
 end
 
 get '/epson' do
-  @labels = LABEL_REPO.all(type: 'escpos')
   erb :epson
 end
 
@@ -264,6 +310,37 @@ post '/epson/raw' do
   redirect '/epson'
 end
 
+get '/labels' do
+  @labels = LABEL_REPO.all
+  erb :labels
+end
+
+get '/labels/:id/edit' do
+  @label = LABEL_REPO.find(params[:id])
+  halt 404, 'unknown label' unless @label
+
+  erb :label_edit
+end
+
+post '/labels/:id' do
+  label = LABEL_REPO.update(params[:id], name: params[:name].to_s, content: params[:content].to_s)
+  halt 404, 'unknown label' unless label
+
+  flash "Saved '#{label['name']}'", 'success'
+  redirect '/labels'
+end
+
+get '/labels/:id/preview.png' do
+  label = LABEL_REPO.find(params[:id])
+  halt 404 unless label && label['type'] == 'zpl'
+
+  png = render_zpl_preview(label['content'])
+  halt 502, 'preview unavailable' unless png
+
+  content_type 'image/png'
+  png
+end
+
 post '/labels/:id/print' do
   label = LABEL_REPO.find(params[:id])
   halt 404, 'unknown label' unless label
@@ -279,16 +356,13 @@ post '/labels/:id/print' do
   rescue => e
     flash "Print failed (#{e.message})", 'danger'
   end
-  redirect_target = label['type'] == 'zpl' ? '/upload' : '/epson'
-  redirect redirect_target
+  redirect '/labels'
 end
 
 post '/labels/:id/delete' do
-  label = LABEL_REPO.find(params[:id])
-  redirect_target = label && label['type'] == 'zpl' ? '/upload' : '/epson'
   LABEL_REPO.delete(params[:id])
   flash 'Label deleted', 'success'
-  redirect redirect_target
+  redirect '/labels'
 end
 
 get '/brother' do