feat: Logbuch-Backup/Restore für Eigner mit Plausible-Events

Vollständiges verschlüsseltes .daagbok.json-Backup inkl. Fotos und GPS; Restore auf gleichem oder neuem Account. Events Backup Exported und Backup Restored mit Anzahlen und Restore-Modus.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/i18n/locales/en.json

@@ -321,7 +321,41 @@
       "deleting_account": "Deleting account…",
       "tour_title": "App tour",
       "tour_desc": "Take a guided walkthrough of the main areas of the app again.",
-      "tour_restart": "Restart tour"
+      "tour_restart": "Restart tour",
+      "backup_title": "Backup & restore",
+      "backup_desc": "Full encrypted backup of this logbook (entries, photos, GPS tracks, crew, vessel). Protected with a backup passphrase — restore on this or a new account.",
+      "backup_export_title": "Create backup",
+      "backup_export_desc": "Downloads all local data as a .daagbok.json file. Keep the file and passphrase separate and secure.",
+      "backup_restore_title": "Restore backup",
+      "backup_restore_desc": "Restores a backup into your current account — including after registering a new account.",
+      "backup_passphrase": "Backup passphrase",
+      "backup_passphrase_placeholder": "At least 8 characters",
+      "backup_passphrase_confirm": "Confirm passphrase",
+      "backup_passphrase_short": "The backup passphrase must be at least 8 characters.",
+      "backup_passphrase_mismatch": "Passphrases do not match.",
+      "backup_wrong_passphrase": "Wrong passphrase or corrupted backup.",
+      "backup_export_btn": "Download backup",
+      "backup_exporting": "Creating backup…",
+      "backup_export_success": "Backup created ({{count}} travel days).",
+      "backup_file_label": "Backup file (.daagbok.json)",
+      "backup_preview_btn": "Verify contents",
+      "backup_previewing": "Verifying…",
+      "backup_restore_btn": "Restore",
+      "backup_restoring": "Restoring…",
+      "backup_restore_success": "Logbook “{{title}}” has been restored.",
+      "backup_restore_cancelled": "Restore cancelled.",
+      "backup_invalid_json": "The file is not valid JSON.",
+      "backup_invalid_format": "Unknown or outdated backup format.",
+      "backup_not_owner": "Only the logbook owner can create backups.",
+      "backup_not_authenticated": "Please sign in to restore a backup.",
+      "backup_id_conflict": "A logbook with this ID already exists.",
+      "backup_overwrite_confirm": "The existing logbook with the same ID will be replaced. Continue?",
+      "backup_new_id_confirm": "Import the backup as a new logbook with a new ID?",
+      "backup_stat_entries": "{{count}} travel days",
+      "backup_stat_photos": "{{count}} photos",
+      "backup_stat_crew": "{{count}} crew records",
+      "backup_stat_tracks": "{{count}} GPS tracks",
+      "backup_exported_at": "Exported: {{date}}"
     },
     "disclaimer": {
       "title": "Important notice",