feat: make action buttons always visible on all tree items

- Remove hover-based opacity transitions for action buttons
- Set action buttons to always visible (opacity: 1, pointer-events: auto)
- Remove desktop hover states and mobile/touch device media queries
- Simplify CSS by eliminating conditional visibility logic

frontend/style.css

@@ -895,8 +895,8 @@ select {
   background: transparent;
   color: var(--text-muted);
   cursor: pointer;
-  opacity: 0;
-  pointer-events: none;
+  opacity: 1;
+  pointer-events: auto;
   transition:
     opacity 140ms ease,
     color 140ms ease,
@@ -913,14 +913,6 @@ select {
   outline: none;
 }
 
-.tree-item:hover .tree-item-action-btn,
-.tree-item:focus-within .tree-item-action-btn,
-.tree-item.path-selected .tree-item-action-btn,
-.tree-item.active .tree-item-action-btn {
-  opacity: 1;
-  pointer-events: auto;
-}
-
 .tree-children {
   padding-left: 16px;
   overflow: hidden;
@@ -944,13 +936,6 @@ select {
   padding: 4px 0 10px 12px;
 }
 
-@media (hover: none), (pointer: coarse), (max-width: 768px) {
-  .tree-item-action-btn {
-    opacity: 1;
-    pointer-events: auto;
-  }
-}
-
 .filter-result-item {
   align-items: flex-start;
   white-space: normal;