feat(11-04): add GET /clients/{id} route and client_detail.html template

- Add client_detail route in pages.py with 404 on unknown client_id
- Build grouped dict {client.name: [printers]} for printer_list partial reuse
- Pass clients + driver_data for edit modal compatibility
- Create client_detail.html extending base.html, includes printer_list partial

imptune/templates/client_detail.html

@@ -0,0 +1,11 @@
+{% extends "base.html" %}
+
+{% block content %}
+<h1>{{ client.name }}</h1>
+<p><a href="/clients">&larr; All Clients</a></p>
+
+<section>
+  <h2>Printers</h2>
+  {% include "partials/printer_list.html" %}
+</section>
+{% endblock %}