Add a direct node search entry to the workflow canvas header

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### 1. Is this request related to a challenge you're experiencing? Tell me about your story.

The existing node search requires `Ctrl/Cmd + K` and `@node`, which is not easy to discover on large Workflow or Chatflow canvases.

Please add a visible “Search nodes” entry in the canvas header to open the existing node search directly.

Related to #38184: it implemented node search and navigation; this request only adds a direct UI entry point.

### 2. Additional context or comments

_No response_

### 3. Can you help us with this feature?

- [x] I am interested in contributing to this feature.

Start by locating the existing node search implementation from related issue #38184 and the workflow or Chatflow canvas header. Add a visible “Search nodes” entry that opens the existing search directly, then verify it works on both canvas types and remains discoverable without Ctrl/Cmd + K or @node.

Written by the indexing model from the issue text.