refactor(file-upload): SSR-first state, click-to-replace, cleaner preview

- Server renders initial file-upload state (src, alt, label text,
  has-current class) so there is no flash of broken-image icon while
  JS boots. JS no longer duplicates that work — it only reacts to
  user input (select, clear, drag-and-drop) and uses data-current-src
  solely to restore the server file after a blob preview.
- Clicking the current preview image now opens the file dialog, mirrors
  the Replace button, with hover affordance (cursor + primary border).
- Drop the transparency checker pattern in the preview — the admin UI
  does not need Figma-style transparency semantics. Preview is now a
  flat theme-aware surface (--app-preview-bg) that keeps white or
  black logos visible against a neutral gray.
- Move tenant favicon out of the aside into the Master-data tab as its
  own details block next to the Tenant-logos section. Uses the same
  barrier-form pattern (HTML5 form attribute + app-file-upload.phtml)
  for a consistent instant-upload UX.
- Tenant-logo slot label is a native <label> element — picks up the
  global form-label typography and spacing, no custom muted-color
  class needed.

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

i18n/default_de.json

@@ -336,6 +336,9 @@
   "Remove": "Entfernen",
   "Allowed file types: SVG, PNG, JPG, WEBP": "Erlaubte Dateitypen: SVG, PNG, JPG, WEBP",
   "Favicon": "Favicon",
+  "Tenant favicon": "Mandanten-Favicon",
+  "Square PNG. Icons are center-cropped.": "Quadratisches PNG. Symbole werden mittig zugeschnitten.",
+  "Delete this favicon?": "Dieses Favicon löschen?",
   "Upload favicon": "Favicon hochladen",
   "Favicon updated": "Favicon aktualisiert",
   "Favicon removed": "Favicon entfernt",