feat(theme): persist user theme preferences (#2756)

- Migrate theme persistence from `sessionStorage` to `localStorage`
- Rename theme HTML attribute to `data-bs-theme` for Bootstrap compatibility
- Trim and compile CSS according to the chosen theme mode

_javascript/modules/components/mode-toggle.js

@@ -1,15 +1,46 @@
 /**
- * Add listener for theme mode toggle
+ * Sets up the mode toggle dropdown, allowing users to switch between light, dark, and system themes.
+ *
+ * Dependencies:
+ *  - Theme (${JS_ROOT}/theme.js)
  */
 
-const $toggle = document.getElementById('mode-toggle');
+import 'bootstrap/js/src/dropdown.js';
+
+const ACTIVE_CLASS = 'active';
+const dropdown = document.querySelector('#mode-toggle + .dropdown-menu');
+const activeMode = Theme.isSystemTheme
+  ? Theme.Mode.SYSTEM
+  : Theme.resolvedTheme;
 
 export function modeWatcher() {
-  if (!$toggle) {
+  if (!Theme.isToggleable) {
     return;
   }
 
-  $toggle.addEventListener('click', () => {
-    Theme.flip();
+  dropdown.querySelectorAll('.dropdown-item').forEach((option) => {
+    const mode = option.dataset.themeMode;
+    if (mode === activeMode) {
+      option.classList.add(ACTIVE_CLASS);
+      return;
+    }
+  });
+
+  dropdown.addEventListener('click', (event) => {
+    const current = event.target.closest('.dropdown-item');
+
+    if (!current) {
+      return;
+    }
+
+    const lastActive = dropdown.querySelector(`.${ACTIVE_CLASS}`);
+
+    if (lastActive === current) {
+      return;
+    }
+
+    lastActive.classList.remove(ACTIVE_CLASS);
+    current.classList.add(ACTIVE_CLASS);
+    Theme.update(current.dataset.themeMode);
   });
 }