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/label_repository.rb

@@ -34,6 +34,16 @@ class LabelRepository
     label
   end
 
+  def update(id, name:, content:)
+    label = find(id)
+    return nil unless label
+
+    label['name'] = name
+    label['content'] = content
+    File.write(path_for(id), JSON.generate(label))
+    label
+  end
+
   def delete(id)
     File.delete(path_for(id)) if File.exist?(path_for(id))
   end