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/index.erb

@@ -1,47 +1,11 @@
-<div class="card" >
+<div class="card">
   <div class="card-body">
-    <h5 class="card-title">Brother Label Designer</h5>
-    <p class="card-text">A web service to print labels on Brother QL label printers.</p>
-    <a href="http://rpi-printserver.local:8560/labeldesigner" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">QL-560</a>
-    <a href="http://rpi-printserver.local:8700/labeldesigner" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">QL-700</a>
-    <a href="http://rpi-printserver.local:9100/labeldesigner" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">QL-1100</a>
-  </div>
-</div>
-<div class="card" >
-  <div class="card-body">
-    <h5 class="card-title">Cockpit</h5>
-    <p class="card-text">Cockpit is a web-based graphical interface for servers, intended for everyone</p>
-    <a href="https://rpi-printserver.local:9090" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">Go</a>
-  </div>
-</div>
-<div class="card" >
-  <div class="card-body">
-    <h5 class="card-title">ZPL Viewer</h5>
-    <p class="card-text">An useful tool for labels in ZPL format.</p>
-    <a href="https://labelary.com/viewer.html" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">Go</a>
-  </div>
-</div>
-<div class="card" >
-  <div class="card-body">
-    <h5 class="card-title">Creality Sonic Pad</h5>
-    <p class="card-text">3D Printer</p>
-    <a href="http://spad-9088.local" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">Fluidd</a>
-    <a href="http://spad-9088.local:8819" class="btn btn-outline-secondary" target="_blank" rel="noopener noreferrer">Mainsail</a>
-    <a href="http://spad-9088.local:7125" class="btn btn-outline-secondary" target="_blank" rel="noopener noreferrer">Moonraker</a>
-  </div>
-</div>
-<div class="card" >
-  <div class="card-body">
-    <h5 class="card-title">Bigtreetech Pad 7 CNC</h5>
-    <p class="card-text">CNC</p>
-    <a href="http://btt-pad7-cnc.local" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">Mainsail</a>
-    <a href="http://btt-pad7-cnc.local:7125" class="btn btn-outline-secondary" target="_blank" rel="noopener noreferrer">Moonraker</a>
-  </div>
-</div>
-<div class="card" >
-  <div class="card-body">
-    <h5 class="card-title">Bigtreetech Pad 7 CAN</h5>
-    <p class="card-text">CAN</p>
-    <a href="http://rpi-btt-pad7-can.local" class="btn btn-secondary" target="_blank" rel="noopener noreferrer">Mainsail</a>
+    <h5 class="card-title">Print server</h5>
+    <p class="card-text">Connected printers:</p>
+    <ul>
+      <li>Datamax-ONeil E4204B Mark III (ZPL, 100x150) -- <a href="/upload">Shipping labels</a></li>
+      <li>Epson TM-T20II (ESC/POS) -- <a href="/epson">Receipts</a></li>
+      <li>Brother QL-560, QL-700, QL-1100 -- <a href="/brother">Brother</a></li>
+    </ul>
   </div>
 </div>