Add Jinja2 web UI with Bootstrap 5 dark theme

- app/templates/: base layout with sidebar nav + 6 pages
  (dashboard, printers, jobs, filaments, todos, pricing)
- app/routers/ui.py: HTML routes querying the DB live
- API routes moved to /api/* prefix
- requirements: jinja2, python-multipart

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/templates/printers.html

@@ -0,0 +1,41 @@
+{% extends "base.html" %}
+{% block title %}Printers — POPS{% endblock %}
+
+{% block content %}
+<h5 class="text-white mb-4">Printers</h5>
+<div class="card">
+  {% if printers %}
+  <table class="table table-dark table-hover mb-0 align-middle">
+    <thead>
+      <tr class="text-secondary small">
+        <th class="fw-normal">Name</th>
+        <th class="fw-normal">Model</th>
+        <th class="fw-normal">Location</th>
+        <th class="fw-normal">Status</th>
+        <th class="fw-normal">Added</th>
+      </tr>
+    </thead>
+    <tbody>
+    {% for p in printers %}
+    <tr>
+      <td class="fw-medium text-white">{{ p.name }}</td>
+      <td class="text-secondary">{{ p.model or '—' }}</td>
+      <td class="text-secondary">{{ p.location or '—' }}</td>
+      <td>
+        {% set s = p.status.value %}
+        {% if   s == 'printing' %}<span class="badge bg-primary">printing</span>
+        {% elif s == 'idle'     %}<span class="badge bg-success">idle</span>
+        {% elif s == 'error'    %}<span class="badge bg-danger">error</span>
+        {% else                 %}<span class="badge bg-secondary">offline</span>
+        {% endif %}
+      </td>
+      <td class="text-secondary small">{{ p.created_at.strftime('%d.%m.%Y') }}</td>
+    </tr>
+    {% endfor %}
+    </tbody>
+  </table>
+  {% else %}
+  <div class="card-body text-secondary">No printers configured yet.</div>
+  {% endif %}
+</div>
+{% endblock %}