feat: add theme selector to unauthenticated pages

assets/css/app.css

@@ -154,6 +154,14 @@
   background-color: var(--color-base-100);
 }
 
+/* WCAG 2.2 AA (4.5:1 for normal text): Form labels. DaisyUI .label uses 60% opacity,
+   which fails contrast. Override to 85% of base-content so labels stay slightly
+   de‑emphasised vs body text but meet the minimum ratio. */
+[data-theme="light"] .label,
+[data-theme="dark"] .label {
+  color: color-mix(in oklab, var(--color-base-content) 85%, transparent);
+}
+
 /* WCAG 2.2 AA (4.5:1 for normal text): Badge text must contrast with badge background.
    Theme tokens *-content are often too light on * backgrounds in light theme, and
    badge-soft uses variant as text on a light tint (low contrast). We override