feat(light-dark): normalize light/dark/like-os mode

permits user to define light, dark or like the desktop

static/js/core/theme-init.js

@@ -1,3 +1,21 @@
-// Apply saved theme immediately (render-blocking) to prevent FOUC.
-// This file MUST be loaded without "defer" or "async".
-if (localStorage.getItem('oxicloud_theme') === 'dark') document.documentElement.setAttribute('data-theme', 'dark');
+// Apply saved colour-scheme choice immediately (render-blocking) to prevent FOUC.
+// This file MUST be loaded WITHOUT "defer" or "async" so it runs before any paint.
+//
+// localStorage values:
+//   'light' / 'dark' → force that mode (stamp html[data-color-scheme]).
+//   'auto'  / absent → follow the OS preference (no attribute → CSS default
+//                      `color-scheme: light dark` resolves via prefers-color-scheme).
+//
+// Backward compat: existing users have 'light' / 'dark' stored from the old
+// binary toggle; both still apply correctly. New 'auto' value is introduced
+// by the segmented control in user-menu.
+
+(() => {
+    var saved = localStorage.getItem('oxicloud_theme');
+    var html = document.documentElement;
+    if (saved === 'light' || saved === 'dark') {
+        html.setAttribute('data-color-scheme', saved);
+    } else {
+        html.removeAttribute('data-color-scheme');
+    }
+})();