feat: add keyboard shortcuts and context menu (Phase 6)

Phase 6 - Enhancements:
- Add useKeyboardShortcuts composable for global shortcuts
- Implement keyboard shortcuts: F2 (rename), Delete, Ctrl+N (new folder)
- Add ContextMenu component with positioning logic
- Implement right-click context menu for tree nodes
- Add rename dialog for nodes
- Support delete with confirmation
- Add "new subfolder" action for folders

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/SessionTreeNode.vue

@@ -13,6 +13,7 @@ interface Props {
 const props = defineProps<Props>()
 const emit = defineEmits<{
   click: [nodeId: string]
+  contextmenu: [nodeId: string, event: MouseEvent]
 }>()
 
 const treeStore = useSessionTreeStore()
@@ -48,6 +49,10 @@ function handleClick() {
   emit('click', props.node.id)
 }
 
+function handleContextMenu(event: MouseEvent) {
+  emit('contextmenu', props.node.id, event)
+}
+
 function handleDragStart(event: DragEvent) {
   if (dragHandlers) {
     event.dataTransfer!.effectAllowed = 'move'
@@ -93,6 +98,7 @@ function handleDragEnd() {
       }"
       :style="{ paddingLeft: `${level * 16 + 8}px` }"
       @click="handleClick"
+      @contextmenu="handleContextMenu"
       @dragstart="handleDragStart"
       @dragover="handleDragOver"
       @dragleave="handleDragLeave"
@@ -145,6 +151,7 @@ function handleDragEnd() {
         :node="child"
         :level="level + 1"
         @click="emit('click', $event)"
+        @contextmenu="(nodeId, event) => emit('contextmenu', nodeId, event)"
       />
     </template>
   </div>