feat(ui): modernize toasts, enhance button feedback, and restructure layouts

- Add global scale-down animation to button active states for tactile feedback
- Reposition toast notifications to bottom-center with updated modern styling
- Split 'Start Downloads' and 'Add to...' actions in AddDownloadsModal
- Overhaul Category Subfolders layout in SettingsView using flexbox for better alignment

src/index.css

@@ -352,7 +352,7 @@
     color: hsl(var(--text-primary));
     font-weight: 500;
     font-size: 13px;
-    transition: background-color 100ms ease;
+    transition: background-color 100ms ease, transform 100ms ease;
     box-shadow: 0 1px 1px hsl(var(--shadow-color));
   }
 
@@ -362,6 +362,7 @@
 
   .app-button:active:not(:disabled) {
     background: hsl(var(--border-color));
+    transform: scale(0.96);
   }
 
   .app-button-primary {
@@ -382,7 +383,7 @@
     justify-content: center;
     border-radius: 6px;
     color: hsl(var(--text-secondary));
-    transition: background-color 100ms ease;
+    transition: background-color 100ms ease, transform 100ms ease;
   }
 
   .app-icon-button:hover:not(:disabled) {
@@ -392,6 +393,7 @@
 
   .app-icon-button:active:not(:disabled) {
     background: hsl(var(--border-color));
+    transform: scale(0.94);
   }
 
   .app-modal-backdrop {
@@ -695,7 +697,7 @@
   }
 
   .add-download-button:active:not(:disabled) {
-    transform: translateY(1px);
+    transform: scale(0.96);
   }
 
   .add-download-button-secondary {