Add minimap for large graph navigation

[Konstantysz]

Problem

Navigating large graphs (50+ nodes) is difficult. Users lose context when zoomed in.

Proposal

Add minimap overlay (like IDE minimaps or game minimaps).

Design

• Small window in corner (configurable: top-right, bottom-right)
• Shows entire graph at reduced scale
• Highlight current viewport with translucent rectangle
• Click minimap to pan main view to that location

Rendering

• Render simplified nodes (just rectangles with colors)
• Skip text/details for performance
• Update at 10 FPS (no need for 60 FPS)

UI

Toggle visibility with View → Minimap or hotkey (M).

Acceptance Criteria

• Graph with 100 nodes renders smoothly in minimap
• Clicking minimap pans main view correctly