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/src/components/StakeHolder.vue

@@ -208,6 +208,37 @@ function setMaxAmount() {
 }
 
 const snatchSelection = useSnatchSelection(demo, taxRate);
+
+// Test helper - only available in dev mode
+if (import.meta.env.DEV) {
+  if (typeof window !== 'undefined') {
+    window.__testHelpers = {
+      fillStakeForm: async (params: { amount: number; taxRate: number }) => {
+        // Validate inputs
+        const minStakeNum = bigInt2Number(minStake.value, 18);
+        if (params.amount < minStakeNum) {
+          throw new Error(`Stake amount ${params.amount} is below minimum ${minStakeNum}`);
+        }
+
+        const maxStakeNum = maxStakeAmount.value;
+        if (params.amount > maxStakeNum) {
+          throw new Error(`Stake amount ${params.amount} exceeds balance ${maxStakeNum}`);
+        }
+
+        if (params.taxRate <= 0 || params.taxRate > 100) {
+          throw new Error(`Tax rate ${params.taxRate} must be between 0 and 100`);
+        }
+
+        // Fill the form
+        stake.stakingAmountNumber = params.amount;
+        taxRate.value = params.taxRate;
+
+        // Wait for reactive updates
+        await new Promise(resolve => setTimeout(resolve, 100));
+      },
+    };
+  }
+}
 </script>
 
 <style lang="sass">