feat(admin): phase 8 — unsaved-changes guard on all detail panels

Add beforeNavigate + isDirty tracking to users/[id], users/new,
groups/[id], groups/new, and tags/[id] edit panels. When a user
navigates away with unsaved changes, the navigation is cancelled and
an inline amber warning banner appears with a Discard button that
resumes navigation. Saving successfully clears the dirty flag.

Add i18n key admin_unsaved_warning (de/en/es).
Add spec files for groups/[id] and tags/[id] panels.

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

frontend/messages/de.json

@@ -160,6 +160,7 @@
 	"admin_tags_select_prompt": "W\u00e4hle ein Schlagwort aus der Liste.",
 	"admin_tag_edit_heading": "Schlagwort: {name}",
 	"admin_tag_updated": "Schlagwort umbenannt.",
+	"admin_unsaved_warning": "Du hast ungespeicherte Änderungen – speichere oder verwerfe, bevor du wechselst.",
 	"admin_btn_edit_tag_label": "Schlagwort bearbeiten",
 	"admin_tag_delete_confirm": "Wirklich löschen? Das Schlagwort wird aus allen Dokumenten entfernt.",
 	"admin_btn_delete_tag_label": "Schlagwort löschen",

frontend/messages/en.json

@@ -160,6 +160,7 @@
 	"admin_tags_select_prompt": "Select a tag from the list.",
 	"admin_tag_edit_heading": "Tag: {name}",
 	"admin_tag_updated": "Tag renamed.",
+	"admin_unsaved_warning": "You have unsaved changes — save or discard before switching.",
 	"admin_btn_edit_tag_label": "Edit tag",
 	"admin_tag_delete_confirm": "Really delete? The tag will be removed from all documents.",
 	"admin_btn_delete_tag_label": "Delete tag",

frontend/messages/es.json

@@ -160,6 +160,7 @@
 	"admin_tags_select_prompt": "Selecciona una etiqueta de la lista.",
 	"admin_tag_edit_heading": "Etiqueta: {name}",
 	"admin_tag_updated": "Etiqueta renombrada.",
+	"admin_unsaved_warning": "Tienes cambios sin guardar — guarda o descarta antes de cambiar.",
 	"admin_btn_edit_tag_label": "Editar etiqueta",
 	"admin_tag_delete_confirm": "¿Realmente eliminar? La etiqueta se eliminará de todos los documentos.",
 	"admin_btn_delete_tag_label": "Eliminar etiqueta",