CI/CD: GitHub Actions for contract tests and frontend build

[alexanderkoh]

Description

No CI/CD exists. Add GitHub Actions for automated testing.

Workflows needed

Contract tests

on: [push, pull_request]
jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: dtolnay/rust-toolchain@stable
      - run: cd contracts && cargo test -p flash-treasury -p flash-campaign-manager-v2

Frontend build

on: [push, pull_request]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: { node-version: '20' }
      - run: npm ci && npx next build

WASM build (optional)

• Build contract WASMs on release tags
• Upload as release artifacts