feat(#68): lead new document form with file upload, all metadata optional

Restructure the "New Document" page so users can save quickly:

- FileSectionNew becomes the first element, redesigned as a prominent
  upload zone with an icon and large click target
- Title field is rendered standalone below the upload zone; it
  auto-populates from the filename (via parseFilename + stripExtension
  fallback) unless the user has already typed something
- All remaining metadata (who/when, description, transcription) moves
  into a collapsible "Weitere Details" section that auto-expands when
  URL prefill data or a form error is present, or when filename parsing
  detects a date/person
- title is no longer required — the form can be saved with only a file
- DescriptionSection gains a `hideTitle` prop for use in this layout
- `form_label_title` translation key no longer carries a hardcoded `*`;
  the asterisk is rendered by the template only when `titleRequired` is
  set (currently only the edit form)
- E2E tests added for all three scenarios from the issue

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -39,7 +39,7 @@
 	"form_placeholder_location": "z.B. Berlin, Wien…",
 	"form_label_sender": "Absender",
 	"form_label_receivers": "Empfänger",
-	"form_label_title": "Titel *",
+	"form_label_title": "Titel",
 	"form_label_tags": "Schlagworte",
 	"form_label_content": "Inhalt",
 	"form_placeholder_content": "Kurze Beschreibung des Inhalts…",
@@ -75,6 +75,7 @@
 	"doc_file_replace_label": "Neue Datei hochladen",
 	"doc_file_replace_note": "(ersetzt die aktuelle Datei)",
 	"doc_current_file_label": "Aktuelle Datei:",
+	"doc_more_details": "Weitere Details",
 	"doc_new_heading": "Neues Dokument",
 	"doc_edit_heading": "Bearbeiten",
 	"doc_section_details": "Details",