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/todos.html

@@ -0,0 +1,40 @@
+{% extends "base.html" %}
+{% block title %}To-Do — POPS{% endblock %}
+
+{% block content %}
+<h5 class="text-white mb-4">To-Do</h5>
+<div class="card">
+  {% if todos %}
+  <table class="table table-dark table-hover mb-0 align-middle">
+    <thead>
+      <tr class="text-secondary small">
+        <th class="fw-normal">Task</th>
+        <th class="fw-normal">Printer</th>
+        <th class="fw-normal text-center">Done</th>
+        <th class="fw-normal text-end">Created</th>
+      </tr>
+    </thead>
+    <tbody>
+    {% for t in todos %}
+    <tr class="{% if t.done %}opacity-50{% endif %}">
+      <td class="{% if t.done %}text-decoration-line-through text-secondary{% else %}text-white{% endif %}">
+        {{ t.title }}
+      </td>
+      <td class="text-secondary small">{{ t.printer.name if t.printer else '—' }}</td>
+      <td class="text-center">
+        {% if t.done %}
+          <i class="bi bi-check-circle-fill text-success"></i>
+        {% else %}
+          <i class="bi bi-circle text-secondary"></i>
+        {% endif %}
+      </td>
+      <td class="text-secondary small text-end">{{ t.created_at.strftime('%d.%m.%Y') }}</td>
+    </tr>
+    {% endfor %}
+    </tbody>
+  </table>
+  {% else %}
+  <div class="card-body text-secondary">No to-do items yet.</div>
+  {% endif %}
+</div>
+{% endblock %}