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/upload.erb

@@ -32,22 +32,4 @@
           </div>
         </form>
 
-        <% if @labels && !@labels.empty? %>
-          <hr class="mt-4">
-          <h5>Saved labels</h5>
-          <div class="list-group">
-            <% @labels.each do |label| %>
-              <div class="list-group-item d-flex justify-content-between align-items-center">
-                <%= label['name'] %>
-                <div class="d-flex gap-2">
-                  <form action="/labels/<%= label['id'] %>/print" method="POST">
-                    <button class="btn btn-sm btn-secondary" type="submit">Print</button>
-                  </form>
-                  <form action="/labels/<%= label['id'] %>/delete" method="POST">
-                    <button class="btn btn-sm btn-outline-danger" type="submit">Delete</button>
-                  </form>
-                </div>
-              </div>
-            <% end %>
-          </div>
-        <% end %>
+        <p class="mt-4"><a href="/labels">See saved labels &rarr;</a></p>