ci(web): enforce coverage floor in CI

[EmeditWeb]

Summary

No linked issue — CI/tooling hardening (Phase 2 quality gates).

Adds vitest coverage thresholds and switches the CI test step to npm run test:coverage so coverage is enforced on every push and PR. The floor is set as a ratchet just below the current baseline, so it fails on regression without turning main red today.

Baseline measured locally: statements 65.62% · branches 59% · functions 70.7% · lines 66.19%.
Thresholds set: statements 60 · branches 50 · functions 65 · lines 60.

This repo is for the React web app only

• My changes are inside src/ — N/A (config + workflow only; no src/ changes)
• I have NOT added Rust, Soroban, or contract code
• I have NOT added React Native or Expo-specific code
• I have NOT hardcoded hex color values
• All icons are from lucide-react only — N/A (no UI change)
• No API calls made directly in page files — N/A (no page change)

Type of change

• CI / tooling (test coverage gate)

Testing

• npm run test:coverage exits 0 with thresholds enforced
• npm run build passes
• npm run lint passes
• Test count unchanged (49 passing) — no tests removed

Mandatory before requesting review

npm run lint, npm test, and npm run build all exit 0 locally. No tests removed.