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.

lib/receipt_builder.rb

@@ -39,6 +39,13 @@ class ReceiptBuilder
     self
   end
 
+  # Appends already-encoded/raw bytes as-is (e.g. pre-parsed ESC/POS command
+  # sequences), bypassing the CP852 text encoding `text` applies.
+  def raw(bytes)
+    @buffer << bytes
+    self
+  end
+
   def align(mode)
     @buffer << case mode
                when :center then ALIGN_CENT