feat(ui): copyable input partial + dynamic copy-target

- New core partial templates/partials/app-input-copy.phtml renders a
  standard label + input with a compact copy button overlaid on the
  right edge of the input. Used for privacy/imprint URLs on the tenant
  form; reusable for any field where a one-click copy is helpful.
- Extend the shared copy-field component (web/js/components/app-copy-field.js)
  with data-copy-target support — the button now reads the target input's
  .value at click-time, so users can edit a field and still copy the
  current value. Static data-copy-value keeps working unchanged.
- New component CSS web/css/components/app-input-copy.css positions the
  button absolute/inset + margin-block:auto (robust vertical centering
  regardless of input height) and uses a descendant selector (0,2,0)
  so the button wins over the global [data-tooltip] position:relative.
- Also register app-input-copy.css + app-tenant-logo.css in core.css
  @import list — they were only in the shared asset group before, which
  default-template admin pages don't load, so tenant-logo styles were
  effectively missing on admin tenant edit (topbar size etc.).
- Document the Copy-to-Clipboard + Copyable Input standards in
  docs/reference-frontend-javascript.md so future consumers don't
  re-invent the markup/selectors.
- File-upload preview: pending-file block restructured to a compact
  list-item row (small square thumb + filename/size stack + clear X)
  and removed the transparency checker pattern on the current-image
  preview in favour of a flat --app-preview-bg surface.
- Tenant edit page title + breadcrumb now show the tenant description
  ("Acme GmbH") instead of the generic "Mandant bearbeiten" when one
  is present.
- i18n: add "Copy to clipboard" / "In Zwischenablage kopieren" across
  both locales.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/js/components/app-copy-field.js

@@ -1,13 +1,13 @@
 /**
- * Copy-to-clipboard button — click writes `data-copy-value` to the clipboard
- * and swaps the button icon to a check for ~1.2s. Self-contained: markup is
- * fully rendered server-side, this component only wires the click handler.
+ * Copy-to-clipboard button — click writes a value to the clipboard and swaps
+ * the button icon to a check for ~1.2s. Self-contained: markup is fully
+ * rendered server-side, this component only wires the click handler.
  *
- *   <button type="button" data-copy-button data-copy-value="max@acme.com"
- *           aria-label="Copy email">
- *     <i class="bi bi-copy" data-copy-icon-idle></i>
- *     <i class="bi bi-check2" data-copy-icon-done></i>
- *   </button>
+ * Static value (read from the button itself):
+ *   <button type="button" data-copy-button data-copy-value="max@acme.com" …>…</button>
+ *
+ * Live input value (resolved at click-time from the target element's .value):
+ *   <button type="button" data-copy-button data-copy-target="privacy_url" …>…</button>
  */
 import { resolveHost } from '../core/app-dom.js';
 
@@ -71,7 +71,14 @@ export function initCopyField(root = document, options = {}) {
 
     const onClick = async (event) => {
       event.preventDefault();
-      const value = button.dataset.copyValue || '';
+      const targetId = (button.dataset.copyTarget || '').trim();
+      let value = button.dataset.copyValue || '';
+      if (targetId) {
+        const target = document.getElementById(targetId);
+        if (target && 'value' in target) {
+          value = /** @type {HTMLInputElement} */ (target).value;
+        }
+      }
       const ok = await copyText(value);
       if (!ok) {return;}
       button.classList.add(DONE_CLASS);