feat(skilltree): add central auth pages with shared components

- Add SkillTreeLogo to shared-branding (emerald green theme)
- Add login, register, forgot-password pages using shared-auth-ui
- Initialize authStore in layout alongside skillStore
- Add shared-auth-ui, shared-i18n, shared-branding dependencies
- German translations as default

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/shared-branding/src/logos/SkillTreeLogo.svelte

@@ -0,0 +1,13 @@
+<script lang="ts">
+	import AppLogo from '../AppLogo.svelte';
+
+	interface Props {
+		size?: number;
+		color?: string;
+		class?: string;
+	}
+
+	let { size = 55, color, class: className = '' }: Props = $props();
+</script>
+
+<AppLogo app="skilltree" {size} {color} class={className} />