feat(geschichten): frontend foundation — canBlogWrite, sanitize util, nav, i18n

- Derives canBlogWrite in +layout.server.ts the same way as canAnnotate.
- Adds Geschichten link to AppNav (desktop + mobile, between Stammbaum and Admin).
- Adds error_geschichte_not_found mapping to errors.ts and translation keys
  for the Geschichten index, detail, editor, and confirmation copy in
  de/en/es.
- Adds isomorphic-dompurify-backed safeHtml() helper with allow-list
  matching the backend OWASP policy (p/br/strong/em/h2/h3/ul/ol/li),
  plus Vitest spec.
- Updates legacy spec test data so the new required canBlogWrite layout
  prop type-checks.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/en.json

@@ -919,6 +919,56 @@
 	"bulk_edit_count_pill": "{count} will be edited",
 
 	"nav_stammbaum": "Family tree",
+	"nav_geschichten": "Stories",
+
+	"error_geschichte_not_found": "The story was not found.",
+
+	"geschichten_index_title": "Stories",
+	"geschichten_new_button": "New story",
+	"geschichten_filter_all_pill": "All",
+	"geschichten_filter_choose_person": "Choose person",
+	"geschichten_filter_aria_label": "Filter by person",
+	"geschichten_empty_for_person": "No stories found for {name}.",
+	"geschichten_empty_no_filter": "There are no published stories yet.",
+	"geschichten_back_to_index": "Back to stories",
+	"geschichten_published_on": "published on {date}",
+	"geschichten_persons_section": "People in this story",
+	"geschichten_documents_section": "Referenced documents",
+	"geschichten_card_heading": "Stories",
+	"geschichten_card_write_action": "+ Write a story",
+	"geschichten_card_attach_action": "+ Attach a story",
+	"geschichten_card_show_all_for_person": "All stories about {name}",
+	"geschichten_card_show_all": "Show all",
+
+	"geschichte_editor_title_placeholder": "Story title",
+	"geschichte_editor_body_placeholder": "Write your story here…",
+	"geschichte_editor_status_draft": "DRAFT",
+	"geschichte_editor_status_published": "PUBLISHED",
+	"geschichte_editor_status_draft_hint": "Not yet visible to readers.",
+	"geschichte_editor_status_published_hint": "Visible to all readers.",
+	"geschichte_editor_save_hint_draft": "All changes are saved as a draft.",
+	"geschichte_editor_save_hint_published": "Changes go live immediately.",
+	"geschichte_editor_save_draft": "Save draft",
+	"geschichte_editor_publish": "Publish",
+	"geschichte_editor_save": "Save",
+	"geschichte_editor_unpublish": "Back to draft",
+	"geschichte_editor_title_required": "Please enter a title.",
+	"geschichte_editor_unsaved_changes": "You have unsaved changes — leave anyway?",
+	"geschichte_editor_personen_heading": "People",
+	"geschichte_editor_personen_hint": "Which historical persons appear in this story?",
+	"geschichte_editor_dokumente_heading": "Documents",
+	"geschichte_editor_dokumente_hint": "Which letters or documents are part of this story?",
+	"geschichte_editor_search_person": "Search person…",
+	"geschichte_editor_search_document": "Search document…",
+	"geschichte_editor_toolbar_bold": "Bold (Ctrl+B)",
+	"geschichte_editor_toolbar_italic": "Italic (Ctrl+I)",
+	"geschichte_editor_toolbar_h2": "Heading",
+	"geschichte_editor_toolbar_h3": "Subheading",
+	"geschichte_editor_toolbar_ul": "Bulleted list",
+	"geschichte_editor_toolbar_ol": "Numbered list",
+
+	"geschichte_delete_confirm_title": "Delete story?",
+	"geschichte_delete_confirm_body": "This action cannot be undone. The story will be permanently deleted and removed from all linked person and document pages.",
 
 	"error_relationship_not_found": "Relationship not found.",
 	"error_circular_relationship": "This relationship would form a cycle.",