fix(ui): library-switch overlay was fully opaque — the 'new page' wipe

The switch overlay intended a 50% dim (bg-opacity-50) but set its
background with an inline style — Tailwind's bg-opacity variable does
not apply to inline styles, so the overlay rendered fully opaque. Every
library switch wiped the material area to a solid sheet (chrome stays
visible: the sidebar sits at z-60 above the overlay's z-50) and
hard-cut back when the overlay dropped — reading exactly as a new page
instead of a fade. The dashboard showed it worst because its switch
holds the overlay longest (sections fetch + awaited week-widget refresh
behind it).

- overlay background is now theme-aware translucent
  (color-mix 55% bg-primary over transparent); the dead bg-opacity-50
  class is gone
- the dim lifts before the fade-in starts, so the new content's fade
  plays fully visible rather than being hard-cut mid-transition

web/src/library-switcher.ts

@@ -27,6 +27,11 @@ export async function switchWithTransition(
 
     await fetchFn();
 
+    // Lift the dim BEFORE the fade-in starts, so the new content's fade
+    // plays fully visible instead of being hard-cut by the overlay
+    // disappearing mid-transition.
+    loading.classList.add("hidden");
+
     container.classList.remove("duration-150");
     container.classList.add("duration-300");
     void container.offsetHeight;