[Frontend] Add emergency QR rendering integrity checks

## Problem
A QR generated from a stale or truncated payload can look valid while resolving to the wrong or incomplete emergency profile.
## Acceptance criteria
- QR payloads are generated from a canonical versioned schema with integrity data.
- The UI verifies decode round-trip before offering download/print.
- Revoked, expired, and oversized payloads are blocked with actionable errors.
- Tests cover round-trip vectors, truncation, encoding, print scaling, and dark mode.

Start by tracing the existing QR generation, decode round-trip, and download/print UI paths; the issue names no files or test paths. Use the acceptance criteria to define the canonical versioned payload and integrity behavior, then cover round-trip, truncation, encoding, print scaling, dark mode, and revoked, expired, and oversized payload errors.

Written by the indexing model from the issue text.