feat(20-03): add virtualized OID tree browser component

- Flatten tree for tanstack/react-virtual virtualization
- Expand/collapse branch nodes with chevron toggle
- Checkbox selection for leaf nodes (selectable OIDs)
- Search filter by name or OID substring
- Expand all / collapse all toolbar buttons
- ARIA tree roles for accessibility

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

frontend/src/components/settings/OIDTreeBrowser.tsx

@@ -0,0 +1,239 @@
+import { useState, useRef, useMemo, useCallback } from 'react'
+import { useVirtualizer } from '@tanstack/react-virtual'
+import { ChevronRight, ChevronsDownUp, ChevronsUpDown, Search } from 'lucide-react'
+import { Checkbox } from '@/components/ui/checkbox'
+import { Button } from '@/components/ui/button'
+import { Input } from '@/components/ui/input'
+import type { OIDNode } from '@/lib/api'
+
+// ─── Types ──────────────────────────────────────────────────────────────────
+
+interface OIDTreeBrowserProps {
+  nodes: OIDNode[]
+  selectedOids: Set<string>
+  onToggleOid: (oid: string, node: OIDNode) => void
+}
+
+interface FlatOIDRow {
+  node: OIDNode
+  depth: number
+  hasChildren: boolean
+  isExpanded: boolean
+}
+
+// ─── Tree Flattening ────────────────────────────────────────────────────────
+
+function flattenTree(
+  nodes: OIDNode[],
+  expandedOids: Set<string>,
+  depth: number = 0,
+): FlatOIDRow[] {
+  const result: FlatOIDRow[] = []
+  for (const node of nodes) {
+    const hasChildren = (node.children?.length ?? 0) > 0
+    const isExpanded = expandedOids.has(node.oid)
+    result.push({ node, depth, hasChildren, isExpanded })
+    if (hasChildren && isExpanded && node.children) {
+      result.push(...flattenTree(node.children, expandedOids, depth + 1))
+    }
+  }
+  return result
+}
+
+function countAllNodes(nodes: OIDNode[]): number {
+  let count = 0
+  for (const node of nodes) {
+    count += 1
+    if (node.children) count += countAllNodes(node.children)
+  }
+  return count
+}
+
+function collectAllOids(nodes: OIDNode[], set: Set<string>): void {
+  for (const node of nodes) {
+    set.add(node.oid)
+    if (node.children) collectAllOids(node.children, set)
+  }
+}
+
+// ─── Component ──────────────────────────────────────────────────────────────
+
+export function OIDTreeBrowser({ nodes, selectedOids, onToggleOid }: OIDTreeBrowserProps) {
+  const scrollRef = useRef<HTMLDivElement>(null)
+  const [expandedOids, setExpandedOids] = useState<Set<string>>(new Set())
+  const [search, setSearch] = useState('')
+
+  const totalNodeCount = useMemo(() => countAllNodes(nodes), [nodes])
+
+  const flatRows = useMemo(
+    () => flattenTree(nodes, expandedOids),
+    [nodes, expandedOids],
+  )
+
+  // Filter rows when search is active
+  const filteredRows = useMemo(() => {
+    if (!search.trim()) return flatRows
+    const term = search.toLowerCase()
+    return flatRows.filter(
+      (row) =>
+        row.node.name.toLowerCase().includes(term) ||
+        row.node.oid.toLowerCase().includes(term),
+    )
+  }, [flatRows, search])
+
+  const virtualizer = useVirtualizer({
+    count: filteredRows.length,
+    getScrollElement: () => scrollRef.current,
+    estimateSize: () => 32,
+    overscan: 10,
+  })
+
+  const toggleExpand = useCallback((oid: string) => {
+    setExpandedOids((prev) => {
+      const next = new Set(prev)
+      if (next.has(oid)) {
+        next.delete(oid)
+      } else {
+        next.add(oid)
+      }
+      return next
+    })
+  }, [])
+
+  const expandAll = useCallback(() => {
+    const allOids = new Set<string>()
+    collectAllOids(nodes, allOids)
+    setExpandedOids(allOids)
+  }, [nodes])
+
+  const collapseAll = useCallback(() => {
+    setExpandedOids(new Set())
+  }, [])
+
+  // ─── Empty state ──────────────────────────────────────────────────────
+
+  if (nodes.length === 0) {
+    return (
+      <div className="py-8 text-center">
+        <p className="text-sm text-text-muted">Upload a MIB file to browse OIDs</p>
+      </div>
+    )
+  }
+
+  // ─── Render ───────────────────────────────────────────────────────────
+
+  return (
+    <div className="space-y-2">
+      {/* Toolbar */}
+      <div className="flex items-center gap-2">
+        <div className="relative flex-1">
+          <Search className="absolute left-2 top-1/2 -translate-y-1/2 h-3.5 w-3.5 text-text-muted" />
+          <Input
+            value={search}
+            onChange={(e) => setSearch(e.target.value)}
+            placeholder="Filter by name or OID..."
+            className="h-7 text-xs pl-7"
+          />
+        </div>
+        <Button variant="ghost" size="sm" className="h-7 text-xs" onClick={expandAll}>
+          <ChevronsUpDown className="h-3.5 w-3.5" /> Expand
+        </Button>
+        <Button variant="ghost" size="sm" className="h-7 text-xs" onClick={collapseAll}>
+          <ChevronsDownUp className="h-3.5 w-3.5" /> Collapse
+        </Button>
+        <span className="text-[10px] text-text-muted whitespace-nowrap">
+          {filteredRows.length} of {totalNodeCount} nodes
+        </span>
+      </div>
+
+      {/* Tree */}
+      <div
+        ref={scrollRef}
+        role="tree"
+        className="h-[400px] overflow-auto border border-border rounded-sm"
+      >
+        <div
+          style={{
+            height: `${virtualizer.getTotalSize()}px`,
+            width: '100%',
+            position: 'relative',
+          }}
+        >
+          {virtualizer.getVirtualItems().map((virtualRow) => {
+            const row = filteredRows[virtualRow.index]
+            const isSelectable = !row.hasChildren || !!row.node.type
+            const isSelected = selectedOids.has(row.node.oid)
+
+            return (
+              <div
+                key={row.node.oid}
+                role="treeitem"
+                aria-expanded={row.hasChildren ? row.isExpanded : undefined}
+                aria-level={row.depth + 1}
+                data-index={virtualRow.index}
+                ref={virtualizer.measureElement}
+                className={`flex items-center gap-2 h-8 hover:bg-surface-hover cursor-pointer ${isSelected ? 'bg-accent/10' : ''}`}
+                style={{
+                  position: 'absolute',
+                  top: 0,
+                  left: 0,
+                  width: '100%',
+                  transform: `translateY(${virtualRow.start}px)`,
+                  paddingLeft: `${row.depth * 20 + 8}px`,
+                }}
+              >
+                {/* Expand/collapse chevron */}
+                {row.hasChildren ? (
+                  <button
+                    type="button"
+                    className="flex-shrink-0 p-0.5 hover:bg-surface-raised rounded"
+                    onClick={(e) => {
+                      e.stopPropagation()
+                      toggleExpand(row.node.oid)
+                    }}
+                  >
+                    <ChevronRight
+                      className={`h-3.5 w-3.5 text-text-muted transition-transform ${row.isExpanded ? 'rotate-90' : ''}`}
+                    />
+                  </button>
+                ) : (
+                  <span className="w-5 flex-shrink-0" />
+                )}
+
+                {/* Checkbox for selectable nodes */}
+                {isSelectable && (
+                  <Checkbox
+                    checked={isSelected}
+                    onCheckedChange={() => onToggleOid(row.node.oid, row.node)}
+                    className="flex-shrink-0"
+                    onClick={(e) => e.stopPropagation()}
+                  />
+                )}
+
+                {/* Name */}
+                <span
+                  className="text-sm text-text-primary font-mono truncate"
+                  title={row.node.description ?? undefined}
+                >
+                  {row.node.name}
+                </span>
+
+                {/* OID */}
+                <span className="text-xs text-text-muted font-mono flex-shrink-0">
+                  {row.node.oid}
+                </span>
+
+                {/* Type badge */}
+                {row.node.type && (
+                  <span className="text-[10px] bg-surface-raised px-1 py-0.5 rounded flex-shrink-0">
+                    {row.node.type}
+                  </span>
+                )}
+              </div>
+            )
+          })}
+        </div>
+      </div>
+    </div>
+  )
+}