feat(10-09): plumb keyboard shortcut ref chain — DropZone→SearchBar→StorageBrowser→FileManagerView

- DropZone: defineExpose({ triggerInput }) to expose file picker trigger
- SearchBar: add inputEl ref + defineExpose({ focus }) for / shortcut
- StorageBrowser: add dropZoneRef + searchBarRef; expand defineExpose to include triggerUpload, focusSearch, clearSearch
- FileManagerView: add defineExpose delegating all four methods to browserRef via optional chaining

frontend/src/views/FileManagerView.vue

@@ -180,4 +180,11 @@ async function doDeleteDoc(docId) {
 function topicColor(name) {
   return topicsStore.topics.find(t => t.name === name)?.color ?? '#6366f1'
 }
+
+defineExpose({
+  focusSearch: () => browserRef.value?.focusSearch?.(),
+  triggerUpload: () => browserRef.value?.triggerUpload?.(),
+  startNewFolder: () => browserRef.value?.startNewFolder?.(),
+  clearSearch: () => browserRef.value?.clearSearch?.(),
+})
 </script>