fix(ux): microinteraction fixes - swipe hint, locale loading, haptics, weather toast, FAB backdrop

- tasks.js: add maybeShowSwipeHint (long loop, max 3x) - matches shopping.js pattern
- tasks.js: vibrate(15) on task status toggle
- oikos-locale-picker: show disabled/loading state for both reload and setLocale paths
- dashboard: show success toast after weather refresh (all 4 locales)
- dashboard: add semi-transparent FAB backdrop to signal open mode

public/styles/dashboard.css

@@ -936,6 +936,21 @@
   background-color: var(--neutral-600);
 }
 
+.fab-backdrop {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.25);
+  z-index: calc(var(--z-nav) - 10);
+  opacity: 0;
+  pointer-events: none;
+  transition: opacity var(--transition-base);
+}
+
+.fab-backdrop--visible {
+  opacity: 1;
+  pointer-events: auto;
+}
+
 .fab-actions {
   display: flex;
   flex-direction: column;