Art: full-image thumbnails, detail page with copy-to-clipboard

- gallery thumbnails now object-fit: contain (whole image, letterboxed),
  and a card links to the new detail page instead of the raw file
- /art/{id} detail page: large uncropped image, "Open full size",
  "Delete" (moved here from the gallery), and "Copy image"
- clipboard Stimulus controller copies the image to the OS clipboard
  (converts non-PNG via canvas) for pasting into WhatsApp etc.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/art/show.html.twig

@@ -0,0 +1,38 @@
+{% extends 'base.html.twig' %}
+
+{% block title %}{{ art.name }} · Art · dmtools{% endblock %}
+
+{% block body %}
+    <div class="d-flex justify-content-between align-items-start mb-3 gap-3 flex-wrap">
+        <div>
+            <a href="{{ path('app_art') }}" class="text-decoration-none small">&larr; Art</a>
+            <h1 class="h3 mb-0">{{ art.name }}</h1>
+            <span class="text-body-secondary small">
+                {{ art.uploadedAt|date('Y-m-d H:i') }} · {{ (art.size / 1024)|round }} KB
+                {%- if art.originalName %} · {{ art.originalName }}{% endif %}
+            </span>
+        </div>
+
+        <div class="d-flex gap-2 flex-wrap">
+            <button type="button" class="btn btn-outline-secondary btn-sm"
+                    data-controller="clipboard"
+                    data-clipboard-url-value="{{ path('app_art_file', {id: art.id}) }}"
+                    data-action="clipboard#copy">Copy image</button>
+
+            <a href="{{ path('app_art_file', {id: art.id}) }}" target="_blank" rel="noopener"
+               class="btn btn-outline-secondary btn-sm">Open full size</a>
+
+            <form method="post" action="{{ path('app_art_delete', {id: art.id}) }}"
+                  onsubmit="return confirm('Delete “{{ art.name|e('js') }}”?')">
+                <input type="hidden" name="_token" value="{{ csrf_token('delete-art-' ~ art.id) }}">
+                <button class="btn btn-outline-danger btn-sm">Delete</button>
+            </form>
+        </div>
+    </div>
+
+    <div class="card shadow-sm">
+        <div class="card-body text-center art-full">
+            <img src="{{ path('app_art_file', {id: art.id}) }}" alt="{{ art.name }}">
+        </div>
+    </div>
+{% endblock %}