feat(web): login reason banner + return-to + empty-field guard (#48)

web/src/i18n/en.json

@@ -1,7 +1,7 @@
 {
   "common": { "yes": "Yes", "no": "No", "close": "Close", "loading": "Loading", "filter": "Filter…", "noMatches": "No matches", "language": "Language", "skipToContent": "Skip to content" },
   "nav": { "objects": "Objects", "vocabularies": "Vocabularies", "authorities": "Authorities", "fields": "Fields", "search": "Search", "collapseSidebar": "Collapse sidebar", "expandSidebar": "Expand sidebar" },
-  "auth": { "email": "Email", "password": "Password", "signIn": "Sign in", "signOut": "Sign out", "invalid": "Invalid email or password", "networkError": "Could not reach the server" },
+  "auth": { "email": "Email", "password": "Password", "signIn": "Sign in", "signOut": "Sign out", "invalid": "Invalid email or password", "networkError": "Could not reach the server", "sessionExpired": "Your session expired — please sign in again.", "signingOut": "Signing out…" },
   "objects": { "title": "Objects", "empty": "No objects yet", "loadError": "Could not load objects", "notFound": "Object not found", "prev": "Previous", "next": "Next", "of": "of", "new": "New object", "filter": "Filter objects…", "pageSize": "Per page", "columns": { "number": "Object №", "name": "Name", "visibility": "Visibility", "location": "Location", "count": "#", "updated": "Updated" }, "unknownRef": "(unknown)" },
   "fieldsLabels": { "objectNumber": "Object number", "objectName": "Name", "count": "Number of objects", "briefDescription": "Brief description", "currentLocation": "Current location", "currentOwner": "Current owner", "recorder": "Recorder", "recordingDate": "Recording date", "visibility": "Visibility" },
   "visibility": { "draft": "Draft", "internal": "Internal", "public": "Public" },