feat: add action buttons and long-press support for vault tree items with mobile-friendly context menus

- Add action buttons (ellipsis icon) to vault, directory, and file tree items
- Implement long-press gesture detection for mobile devices with 550ms delay and 10px movement threshold
- Show action buttons on hover for desktop and always visible on mobile/touch devices
- Position context menus near action buttons to prevent off-screen rendering
- Prevent click events from firing after long-press ges

frontend/style.css

@@ -837,6 +837,9 @@ select {
   background: var(--bg-hover);
   color: var(--text-primary);
 }
+.tree-item > .badge-small {
+  margin-left: auto;
+}
 .tree-item.active {
   background: var(--bg-hover);
   color: var(--accent);
@@ -880,6 +883,44 @@ select {
   color: var(--accent-green);
 }
 
+.tree-item-action-btn {
+  margin-left: auto;
+  width: 24px;
+  height: 24px;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  border: 1px solid transparent;
+  border-radius: 6px;
+  background: transparent;
+  color: var(--text-muted);
+  cursor: pointer;
+  opacity: 0;
+  pointer-events: none;
+  transition:
+    opacity 140ms ease,
+    color 140ms ease,
+    background 140ms ease,
+    border-color 140ms ease;
+  flex-shrink: 0;
+}
+
+.tree-item-action-btn:hover,
+.tree-item-action-btn:focus-visible {
+  background: color-mix(in srgb, var(--accent) 10%, transparent);
+  border-color: color-mix(in srgb, var(--accent) 25%, var(--border));
+  color: var(--accent);
+  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;
@@ -903,6 +944,13 @@ 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;