feat: initialise Vue 3 SPA template

- Add Tailwind CSS v4 with light/dark theme support
- Add layout system (default, sidebar, blank)
- Add mock authentication with Pinia store and route guards
- Add error pages (404, 500) and example views
- Add Vitest with 10 example tests across stores
- Configure Prettier formatting and ESLint linting

src/components/ThemeToggle.vue

@@ -0,0 +1,16 @@
+<script setup lang="ts">
+import { useTheme } from "@/stores/theme"
+
+const { mode, toggle } = useTheme()
+</script>
+
+<template>
+  <button
+    type="button"
+    class="cursor-pointer rounded-md border border-border bg-surface-alt p-2 text-foreground hover:bg-border transition-colours"
+    :aria-label="mode === 'dark' ? 'Switch to light mode' : 'Switch to dark mode'"
+    @click="toggle">
+    <span v-if="mode === 'dark'" aria-hidden="true">☀️</span>
+    <span v-else aria-hidden="true">🌙</span>
+  </button>
+</template>