feat: 2D/3D view toggle

[TechPMRyan]

Summary

Add a toggle to switch between the current 3D force graph and a 2D flat layout.

Why

The 3D view earns its keep on first render — it communicates scale and cluster structure well and makes a strong first impression. But when an engineer is doing precision work — tracing a dependency chain, mapping blast radius on a specific node, following a path from controller to repository — 3D gets in the way. Nodes occlude each other, depth perception makes connections ambiguous, and the layout resettles on every load.

2D is the mode for analytical work. Cleaner layout, unambiguous connections, stable positions, easier to follow a specific path. Both modes have a legitimate place depending on what the user is trying to do.

Implementation notes

• The force-graph 2D library (same author as 3d-force-graph, vasturiano) has a near-identical API — this is an incremental implementation, not a rewrite
• Toggle button in the topbar (e.g. 3D / 2D)
• Preserve node selection, search state, and kind filters when switching modes
• Persist last-used mode in localStorage
• 2D layout should use a stable force simulation so layout is consistent between sessions

Acceptance criteria

• Toggle switches between 3D and 2D rendering without page reload
• Node colors, hotspot detection, kind filters, and search all work identically in both modes
• Panel behavior (click to inspect, chip navigation) works identically in both modes
• Selected mode persists across sessions via localStorage
• Both modes tested — layout does not break at 10 nodes or 200 nodes

Priority

Roadmap item — after current 3D UI work is stable. Not a v0.1 blocker.