refactor(ui): unify and modernize in-app toast notifications

- Created root-level ToastProvider and useToast hook
- Replaced scattered ad-hoc toast states with unified provider
- Updated Settings, Scheduler, Speed Limiter, App, and DownloadTable to use ToastContext
- Added variant support (success, info, warning, error) with distinct styling
- Refined toast animations and styling for modern aesthetics
- Adjusted auto-dismiss behavior to ignore actionable or important errors

src/index.css

@@ -875,15 +875,8 @@
     filter: none;
   }
 
-  .app-toast {
-    border: 1px solid hsl(var(--border-modal));
-    border-radius: 8px;
-    background: hsl(var(--surface-overlay));
-    backdrop-filter: blur(20px);
-    -webkit-backdrop-filter: blur(20px);
-    color: hsl(var(--text-primary));
-    box-shadow: 0 4px 12px hsl(var(--shadow-color));
-    animation: toast-in 200ms ease-out;
+  .app-toast-item {
+    animation: toast-in 300ms cubic-bezier(0.2, 0.8, 0.2, 1);
   }
 
   .app-shell {
@@ -1792,8 +1785,8 @@
 }
 
 @keyframes toast-in {
-  from { opacity: 0; transform: translate(-50%, 8px); }
-  to { opacity: 1; transform: translate(-50%, 0); }
+  from { opacity: 0; transform: translateY(12px) scale(0.98); }
+  to { opacity: 1; transform: translateY(0) scale(1); }
 }
 
 @media (prefers-reduced-motion: reduce) {