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/css/components/app-file-upload.css

@@ -169,11 +169,11 @@
   }
 
   /* ── Pending file preview (visible when new file selected) ──
-     Same layout as current: full-width thumb + info row below. */
+     Compact row: small thumb (left) + filename/size stack (middle) + clear X (right). */
   .app-file-upload-preview {
     display: none;
-    flex-direction: column;
-    gap: calc(var(--app-spacing) * 0.5);
+    align-items: center;
+    gap: calc(var(--app-spacing) * 0.75);
     padding: calc(var(--app-spacing) * 0.75);
   }
 
@@ -182,53 +182,50 @@
   }
 
   .app-file-upload-thumbnail {
-    width: 100%;
-    aspect-ratio: 16 / 9;
-    padding: var(--app-spacing);
+    width: 48px;
+    height: 48px;
     border-radius: calc(var(--app-border-radius) * 0.5);
-    object-fit: contain;
-    object-position: center;
+    object-fit: cover;
     border: 1px solid var(--app-muted-border-color);
     background-color: var(--app-preview-bg);
-    box-sizing: border-box;
+    flex-shrink: 0;
   }
 
   .app-file-upload-file-icon {
-    width: 100%;
-    aspect-ratio: 16 / 9;
+    width: 48px;
+    height: 48px;
     border-radius: calc(var(--app-border-radius) * 0.5);
     display: inline-flex;
     align-items: center;
     justify-content: center;
-    font-size: var(--text-3xl);
+    font-size: var(--text-xl);
     color: var(--app-muted-color);
-    background: var(--app-background-color);
+    background: var(--app-preview-bg);
     border: 1px solid var(--app-muted-border-color);
+    flex-shrink: 0;
   }
 
   .app-file-upload-meta {
     display: flex;
-    align-items: center;
-    justify-content: space-between;
-    gap: calc(var(--app-spacing) * 0.5);
+    flex-direction: column;
+    gap: 2px;
+    flex: 1;
     min-width: 0;
   }
 
   .app-file-upload-filename {
     font-size: var(--text-sm);
     font-weight: var(--font-medium);
-    color: var(--app-contrast);
+    color: var(--app-color);
     min-width: 0;
     overflow: hidden;
     text-overflow: ellipsis;
     white-space: nowrap;
-    flex: 1;
   }
 
   .app-file-upload-filesize {
     font-size: var(--text-xs);
     color: var(--app-muted-color);
-    flex-shrink: 0;
   }
 
   .app-file-upload-clear {