Redesign: clear active nav state, Shipping labels + Receipts get free-text/raw print, shared label repository

- Home page is now just the print server overview, no more unrelated
  network links (3D printers, Cockpit) left over from when this was a
  general landing page.
- Navbar shows which section is active; Brother's per-printer buttons
  now live under a dedicated Brother nav item instead of always-visible.
- Shipping labels: paste raw ZPL directly (no need to save to a file
  first), with a Labelary preview link alongside it.
- Receipts: paste already-formatted ESC/POS text directly, printed as-is.
- Both can save what they just printed into a shared label repository
  (lib/label_repository.rb, one JSON file per label under data/labels/)
  for reprinting later -- saving happens independently of whether the
  print itself succeeds, so a failed/offline printer doesn't lose the
  content.

views/upload.erb

@@ -9,3 +9,45 @@
         <form action="/test-print" method="POST" class="mt-3">
           <button class="btn btn-outline-secondary" type="submit">Print test label</button>
         </form>
+
+        <hr class="mt-4">
+
+        <h5>Free text ZPL</h5>
+        <p>
+          <a href="https://labelary.com/viewer.html" target="_blank" rel="noopener noreferrer">Labelary ZPL viewer</a>
+          (preview/build ZPL before pasting it here)
+        </p>
+        <form action="/shipping/zpl" method="POST">
+          <div class="mb-3">
+            <textarea name="zpl" class="form-control" rows="8" placeholder="^XA...^XZ" style="font-family: monospace;"></textarea>
+          </div>
+          <div class="row g-2 align-items-end">
+            <div class="col-auto">
+              <label for="save_as_zpl" class="form-label">Save as (optional)</label>
+              <input name="save_as" class="form-control" type="text" id="save_as_zpl" placeholder="label name" />
+            </div>
+            <div class="col-auto">
+              <button class="btn btn-secondary" type="submit">Print</button>
+            </div>
+          </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 %>