feat(scripts): start-preview-server.sh — profile-aware preview server launcher (Phase 2)

Bridges H2 freeze → live local app gap (DEMO-STORYBOARD.md L1:50–2:00).
Auto-detects profile (docker-compose.yml vs apps/{api,web}/package.json),
picks free port from 18080+, spawns dev servers in background, opens browser.
Idempotent (re-runs only re-open URL), supports stop + status subcommands.
PF_PREVIEW_DRY_RUN=1 keeps unit tests light.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sgwannabe