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>

i18n/default_de.json

@@ -604,6 +604,7 @@
   "English": "Englisch",
   "Switch tenant": "Mandant wechseln",
   "Copy": "Kopieren",
+  "Copy to clipboard": "In Zwischenablage kopieren",
   "Registration successful! Please check your email for the verification code.": "Registrierung erfolgreich! Bitte prüfe deine E-Mails für den Bestätigungscode.",
   "Registration is currently disabled": "Registrierung ist derzeit deaktiviert.",
   "A new verification code has been sent.": "Ein neuer Bestätigungscode wurde gesendet.",

i18n/default_en.json

@@ -604,6 +604,7 @@
   "English": "English",
   "Switch tenant": "Switch tenant",
   "Copy": "Copy",
+  "Copy to clipboard": "Copy to clipboard",
   "Registration successful! Please check your email for the verification code.": "Registration successful! Please check your email for the verification code.",
   "Registration is currently disabled": "Registration is currently disabled.",
   "A new verification code has been sent.": "A new verification code has been sent.",