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>

assets/styles/app.css

@@ -89,3 +89,26 @@ table.dataTable {
 .ft-amp {
     color: var(--bs-secondary-color);
 }
+
+/* --- Art gallery ------------------------------------------------------- */
+.art-thumb {
+    height: 180px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 0.4rem;
+    background: var(--bs-tertiary-bg);
+    border-bottom: 1px solid var(--bs-border-color);
+    border-top-left-radius: inherit;
+    border-top-right-radius: inherit;
+    overflow: hidden;
+}
+.art-thumb img {
+    max-width: 100%;
+    max-height: 100%;
+    object-fit: contain;   /* whole image, never cropped */
+}
+.art-full img {
+    max-width: 100%;
+    height: auto;
+}