Add design principles section to README

[jasoneplumb]

Summary

Adds a new Design Principles section to the README between Features and Tech Stack that outlines the six core architectural decisions:

• Offline-first — App works without connectivity; all tiles cached, no required server calls
• Local-only data — No server storage, no accounts, no telemetry — your trail stays on your device
• Progressive enhancement — Core features work everywhere; advanced features activate when APIs available
• Mobile-native UX — Bottom sheets, safe-area insets, touch controls, and responsive layouts built in
• Minimal dependencies — No framework, no CSS library, no state management — just Leaflet, ESRI Leaflet, and a 400-line app
• Transparent architecture — Single AppState object, explicit data flow, no event bus or indirection

These principles are verified against the actual codebase patterns and dependencies.

Closes #110