PocketPilot-v1

Add detailed self-hosting guide to README

Covers Railway backend deploy, EAS Android build, cost breakdown
with reduction tips, and a known hiccups section from real deployment
experience (Railway root dir, OAuth redirect, blank screen, XHR fix,
EAS env vars, APK reinstall requirement).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>