fix(#221): reset parent/color/delete state when navigating between tag edit pages

SvelteKit reuses the same +page.svelte instance on client-side navigation,
so $state() initialisations only run on mount. Add an $effect keyed on
data.tag.id to reset parentId, selectedColor and deleteConfirmName whenever
the user switches to a different tag in the admin sidebar.

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

frontend/src/routes/admin/tags/[id]/+page.svelte

@@ -21,6 +21,16 @@ function getInitialColor() {
 let parentId = $state(getInitialParentId());
 let selectedColor = $state(getInitialColor());
 
+// SvelteKit reuses the same component instance when navigating between tags client-side.
+// $state() only initialises on mount, so we need an effect to reset local form state
+// whenever the server switches to a different tag.
+$effect(() => {
+	void data.tag.id; // declare dependency
+	parentId = data.tag.parentId ?? '';
+	selectedColor = data.tag.color ?? '';
+	deleteConfirmName = '';
+});
+
 const colors = [
 	'sage',
 	'sienna',