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/labels.erb

@@ -0,0 +1,39 @@
+<h4>Saved labels &amp; receipts</h4>
+
+<% if @labels.empty? %>
+  <p>Nothing saved yet -- use "Save as" on <a href="/upload">Shipping labels</a> or <a href="/epson">Receipts</a>.</p>
+<% else %>
+  <div class="list-group">
+    <% @labels.each do |label| %>
+      <div class="list-group-item">
+        <div class="d-flex justify-content-between align-items-start">
+          <div>
+            <% 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 %>
+            <strong class="ms-2"><%= label['name'] %></strong>
+          </div>
+          <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>
+            <a href="/labels/<%= label['id'] %>/edit" class="btn btn-sm btn-outline-secondary">Edit</a>
+            <form action="/labels/<%= label['id'] %>/delete" method="POST">
+              <button class="btn btn-sm btn-outline-danger" type="submit">Delete</button>
+            </form>
+          </div>
+        </div>
+
+        <div class="mt-2">
+          <% if label['type'] == 'zpl' %>
+            <img src="/labels/<%= label['id'] %>/preview.png" alt="preview" style="max-height: 150px;" onerror="this.remove()" />
+          <% else %>
+            <pre class="bg-light p-2 mb-0" style="max-height: 150px; overflow: auto;"><%= label['content'] %></pre>
+          <% end %>
+        </div>
+      </div>
+    <% end %>
+  </div>
+<% end %>