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-input-copy.css

@@ -0,0 +1,73 @@
+@layer components {
+  /* Copyable input — absolute-positioned button overlay on the right edge
+     of a native-width input. Wrapper is a plain block so it fits into
+     any grid cell without overflow; button is positioned absolute and
+     vertically centered via the inset + margin-block:auto pattern. */
+  .app-input-copy {
+    position: relative;
+  }
+
+  .app-input-copy > input {
+    padding-right: 2.5rem;
+    width: 100%;
+    display: block;
+  }
+
+  /* High specificity (0,2,0) to beat [data-tooltip] (0,1,0) which sets
+     position: relative on the copy button (tooltip host). */
+  .app-input-copy > .app-input-copy-button {
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    right: 0.3rem;
+    margin-block: auto;
+    height: fit-content;
+    background: transparent;
+    border: 0;
+    box-shadow: none;
+    color: var(--app-muted-color);
+    cursor: pointer;
+    padding: 0.3rem 0.45rem;
+    width: auto;
+    line-height: var(--leading-none);
+    border-radius: calc(var(--app-border-radius) * 0.5);
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    transition: color 0.15s ease, background-color 0.15s ease;
+  }
+
+  .app-input-copy > .app-input-copy-button:hover {
+    color: var(--app-primary);
+    background: color-mix(in srgb, var(--app-primary) 10%, transparent);
+  }
+
+  .app-input-copy > .app-input-copy-button:focus-visible {
+    outline: 2px solid var(--app-primary);
+    outline-offset: 1px;
+  }
+
+  .app-input-copy > .app-input-copy-button [data-copy-icon-done] {
+    display: none;
+  }
+
+  .app-input-copy > .app-input-copy-button.is-copied {
+    color: var(--app-action-success-border);
+    background: color-mix(in srgb, var(--app-action-success-border) 12%, transparent);
+  }
+
+  .app-input-copy > .app-input-copy-button.is-copied [data-copy-icon-idle] {
+    display: none;
+  }
+
+  .app-input-copy > .app-input-copy-button.is-copied [data-copy-icon-done] {
+    display: inline-flex;
+  }
+
+  /* Input inside the wrapper is now the last child of label (no longer
+     a direct <input> child). Add the margin-top that the global
+     label > input rule would have applied. */
+  label > .app-input-copy {
+    margin-top: calc(var(--app-spacing) * 0.25);
+  }
+}