feat: add light/dark theme toggle with system preference detection

- Implement theme store with localStorage persistence
- Add ThemeToggle component with animated icon transitions
- Update color system with semantic tokens for light/dark modes
- Add blocking script in app.html to prevent FOUC
- Apply theme-aware styling across all public and admin pages

web/src/lib/components/ThemeToggle.svelte

@@ -0,0 +1,25 @@
+<script lang="ts">
+  import { themeStore } from "$lib/stores/theme.svelte";
+  import IconSun from "~icons/lucide/sun";
+  import IconMoon from "~icons/lucide/moon";
+</script>
+
+<button
+  type="button"
+  onclick={() => themeStore.toggle()}
+  aria-label={themeStore.isDark ? "Switch to light mode" : "Switch to dark mode"}
+  class="relative size-9 rounded-md border border-zinc-300 dark:border-zinc-700 bg-zinc-100 dark:bg-zinc-900/50 hover:bg-zinc-200 dark:hover:bg-zinc-800/70 transition-all duration-200"
+>
+  <div class="absolute inset-0 flex items-center justify-center">
+    <IconSun
+      class="size-5 text-zinc-600 dark:text-zinc-400 transition-all duration-300 {themeStore.isDark
+        ? 'rotate-90 scale-0 opacity-0'
+        : 'rotate-0 scale-100 opacity-100'}"
+    />
+    <IconMoon
+      class="absolute size-5 text-zinc-600 dark:text-zinc-400 transition-all duration-300 {themeStore.isDark
+        ? 'rotate-0 scale-100 opacity-100'
+        : '-rotate-90 scale-0 opacity-0'}"
+    />
+  </div>
+</button>