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.

views/label_edit.erb

@@ -0,0 +1,20 @@
+<h4>Edit label</h4>
+
+<% if @label['type'] == 'zpl' %>
+  <span class="badge bg-primary">ZPL &middot; Datamax</span>
+<% else %>
+  <span class="badge bg-info text-dark">ESC/POS &middot; Epson</span>
+<% end %>
+
+<form action="/labels/<%= @label['id'] %>" method="POST" class="mt-3">
+  <div class="mb-3">
+    <label for="name" class="form-label">Name</label>
+    <input name="name" class="form-control" type="text" id="name" value="<%= @label['name'] %>" />
+  </div>
+  <div class="mb-3">
+    <label for="content" class="form-label">Content</label>
+    <textarea name="content" class="form-control" id="content" rows="12" style="font-family: monospace;"><%= @label['content'] %></textarea>
+  </div>
+  <button class="btn btn-secondary" type="submit">Save</button>
+  <a href="/labels" class="btn btn-outline-secondary">Cancel</a>
+</form>