Refactor XState to simpler state management

[vgeorge]

The current XState implementation is overkill for our simple UI interactions.

Current state: 3-state machine with 6 event types for basic interactions:

• Bbox selection workflow (start → end → complete)
• Map click/hover events
• Toolbar button states
• Side panel visibility
• Feature highlighting

Proposal: Evaluate simpler alternatives (Zustand, XState store, or React state) that better match the UI complexity. Create a proof-of-concept branch, migrate incrementally, and use existing Playwright tests to verify all interactions work.

Goal: Lower barrier to entry for JS developers while maintaining all functionality.