Flow builder: search / filter box in the Add-node palette

[os-zhuang]

Problem

The Add-node palette is a ~20-item scrolling list across 5 categories (Data / Logic / Human / Integration / Flow). Finding the bottom ones (Screen, End, Subflow, Wait) needs scrolling, and the palette is server-driven (GET /api/v1/automation/actions — plugins contribute nodes), so it only grows.

Proposal

A search box at the top of the palette that filters items by label + hint as you type (e.g. scr → Screen, http → HTTP). Nice-to-haves: a "Recently used" group, and keyboard navigation (↑/↓ + Enter).

Acceptance criteria

• Typing filters the palette across all categories (matches label and hint, case-insensitive).
• Empty query shows the full grouped list (current behavior).
• Keyboard: arrow keys move, Enter inserts the highlighted node.
• Works with the server-merged palette (plugin nodes included).

Pointers

• previews/flow-canvas-parts.tsx — NodePalette component + NODE_PALETTE.
• previews/useFlowNodePalette.ts — useFlowNodePalette() supplies the merged item list.