feat(admin/tags): add tags entity with master-detail sub-routes and type-to-confirm delete

Creates the full tags section under /admin/tags/:
- +layout.server.ts: loads tags list via GET /api/tags
- TagsListPanel.svelte: left list panel (name, active state)
- +layout.svelte: composes list panel + children slot
- +page.svelte: empty selection prompt
- [id]/+page.server.ts: rename (PUT) and delete actions
- [id]/+page.svelte: rename form + danger zone with type-to-confirm delete

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

frontend/messages/en.json

@@ -155,6 +155,11 @@
 	"admin_multiselect_hint_full": "Ctrl+Click for multiple selection",
 	"admin_section_tags": "Tags",
 	"admin_tags_warning": "Warning: Renaming or deleting affects all linked documents.",
+	"admin_tags_list_title": "All Tags",
+	"admin_tags_empty": "No tags found.",
+	"admin_tags_select_prompt": "Select a tag from the list.",
+	"admin_tag_edit_heading": "Tag: {name}",
+	"admin_tag_updated": "Tag renamed.",
 	"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",