Enable dark mode

src/lib/components/ThemeToggle.svelte

@@ -0,0 +1,65 @@
+<script lang="ts">
+  import { themeState, toggleTheme } from "$lib/theme.svelte";
+
+  type Props = { size?: "sm" | "md" };
+  let { size = "sm" }: Props = $props();
+
+  const isDark = $derived(themeState.theme === "dark");
+  const iconSize = $derived(size === "md" ? "h-6 w-6" : "h-5 w-5");
+  const padding = $derived(size === "md" ? "p-3" : "p-2.5");
+  const label = $derived(
+    isDark ? "Switch to light theme" : "Switch to dark theme",
+  );
+  // The tooltip clarifies whether the current state follows the OS or is pinned,
+  // so users can tell at a glance if their pick will outlive an OS theme change.
+  const title = $derived(
+    themeState.linked
+      ? `Following system (${themeState.theme})`
+      : `${themeState.theme.charAt(0).toUpperCase()}${themeState.theme.slice(1)} theme`,
+  );
+</script>
+
+<button
+  type="button"
+  onclick={toggleTheme}
+  aria-label={label}
+  aria-pressed={isDark}
+  {title}
+  class="inline-flex items-center justify-center rounded {padding} text-neutral-500 transition-colors hover:bg-neutral-200 hover:text-neutral-700 dark:hover:text-neutral-300 dark:text-neutral-400 dark:hover:bg-neutral-700"
+>
+  {#if isDark}
+    <!-- Moon -->
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      class={iconSize}
+      fill="none"
+      viewBox="0 0 24 24"
+      stroke="currentColor"
+      stroke-width="1.8"
+      aria-hidden="true"
+    >
+      <path
+        stroke-linecap="round"
+        stroke-linejoin="round"
+        d="M21.752 15.002A9.718 9.718 0 0 1 18 15.75c-5.385 0-9.75-4.365-9.75-9.75 0-1.33.266-2.597.748-3.752A9.753 9.753 0 0 0 3 11.25C3 16.635 7.365 21 12.75 21a9.753 9.753 0 0 0 9.002-5.998Z"
+      />
+    </svg>
+  {:else}
+    <!-- Sun -->
+    <svg
+      xmlns="http://www.w3.org/2000/svg"
+      class={iconSize}
+      fill="none"
+      viewBox="0 0 24 24"
+      stroke="currentColor"
+      stroke-width="1.8"
+      aria-hidden="true"
+    >
+      <path
+        stroke-linecap="round"
+        stroke-linejoin="round"
+        d="M12 3v2.25m6.364.386-1.591 1.591M21 12h-2.25m-.386 6.364-1.591-1.591M12 18.75V21m-4.773-4.227-1.591 1.591M5.25 12H3m4.227-4.773L5.636 5.636M15.75 12a3.75 3.75 0 1 1-7.5 0 3.75 3.75 0 0 1 7.5 0Z"
+      />
+    </svg>
+  {/if}
+</button>