Upload PDFs to the library, link them from any link target, show them in a viewer

PDFs go through the same upload field as images, are checked by their content and kept
unchanged in their own table. Every link field can pick one, and a page that links a PDF
opens it in a dialog with the browser's own viewer; touch devices and browsers without a
viewer get a new tab instead.

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

public/assets/admin.css

@@ -975,3 +975,5 @@ details.card > summary:hover { background: #fafbf8 }
 .theme-save .muted { font-size: 12px }
 .theme-form .button-defaults { margin: 0; background: var(--paper) }
 @media (max-width: 1000px) { .type-layout { grid-template-columns: 1fr } }
+/* Ein Dokument in der Mediathek: drei Buchstaben statt eines Zeichens. */
+.row-icon-text { font-size: 11px; font-weight: 700; letter-spacing: .04em }