feat(#221): add parent selector and color picker to admin tag edit form

Tag edit form gains a parent <select> listing all other tags (self
excluded) and a 10-swatch color picker that is only shown when no
parent is selected. Submitting passes parentId and color to the PUT
/api/tags/{id} endpoint via TagUpdateDTO.

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

frontend/src/routes/page.svelte.spec.ts

@@ -31,7 +31,8 @@ const emptyData = {
 		tags: [],
 		sort: 'DATE' as const,
 		dir: 'desc' as const,
-		tagQ: ''
+		tagQ: '',
+		tagOp: 'AND'
 	},
 	documents: [],
 	total: 0,