docs: refresh screenshot for updated graph panel

The extension screenshot now reflects the current graph panel
presentation after the recent query-linking and panel UX changes.

Constraint: Marketplace/readme visuals should match the shipped webview
Rejected: Keep the older screenshot | no longer matched current panel behavior
Confidence: high
Scope-risk: narrow
Directive: Update this asset whenever graph grouping, minimap, or panel controls change materially
Tested: Visual inspection of updated screenshot asset
Not-tested: Marketplace rendering after publish

Author: fe-dudu