feat: Add test helper for E2E staking flow (#66)

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

web-app/README.md

@@ -31,3 +31,56 @@ npm run dev
 ```sh
 npm run build
 ```
+
+## Testing
+
+### Test Helpers
+
+The application exposes test helpers on `window.__testHelpers` in development mode to facilitate E2E testing.
+
+#### Available Helpers
+
+##### `fillStakeForm(params)`
+
+Programmatically fills the staking form without requiring fragile UI selectors.
+
+**Parameters:**
+- `amount` (number): Amount of KRK tokens to stake (must be >= minimum stake)
+- `taxRate` (number): Tax rate percentage (must be between 0 and 100)
+
+**Example:**
+```typescript
+// In Playwright test
+await page.evaluate(async () => {
+  await window.__testHelpers.fillStakeForm({
+    amount: 100,
+    taxRate: 5.0,
+  });
+});
+
+// Then click the stake button
+const stakeButton = page.getByRole('button', { name: /Stake|Snatch and Stake/i });
+await stakeButton.click();
+```
+
+**Validation:**
+- Throws if amount is below minimum stake
+- Throws if amount exceeds wallet balance
+- Throws if tax rate is outside valid range (0-100)
+
+**TypeScript Support:**
+Type declarations are available in `env.d.ts`:
+```typescript
+interface Window {
+  __testHelpers?: {
+    fillStakeForm: (params: { amount: number; taxRate: number }) => Promise<void>;
+  };
+}
+```
+
+**Security:**
+Test helpers are only available when `import.meta.env.DEV === true` and are automatically stripped from production builds.
+
+### E2E Tests
+
+See `INTEGRATION_TEST_STATUS.md` in the repository root for complete testing documentation.