Implements window.__testHelpers.fillStakeForm() to enable stable E2E testing
of the staking form without fragile UI selectors.
## Changes
- Add window.__testHelpers interface (dev mode only)
- Implement fillStakeForm() in StakeHolder.vue with input validation
- Add TypeScript declarations in env.d.ts
- Update E2E test to use helper and verify full user journey
- Create INTEGRATION_TEST_STATUS.md documenting test coverage
- Document helper in web-app/README.md
## Test Coverage
Playwright E2E now validates complete flow:
- Mint ETH via cheats page UI
- Swap KRK via cheats page UI
- Stake KRK via stake page UI (helper + click)
- Verify position via GraphQL
Both Playwright and verify-swap.sh tests now work independently.
resolves#62
Co-authored-by: johba <johba@harb.eth>
Reviewed-on: https://codeberg.org/johba/harb/pulls/66
