subscribe() accepts zero, negative, or invalid subscription periods

### Describe the bug

subscribe() validates recurringCharge and subscriptionOwner, but it does not clearly validate periodInDays or overridePeriodInSecondsForTestnet before creating spend-permission typed data. This can allow invalid subscription periods such as 0, negative numbers, non-integers, or unsafe numbers to reach wallet signing / typed-data generation.

### Steps

1. Open packages/account-sdk/src/interface/payment/subscribe.ts
2. Check subscribe()
3. Notice that recurringCharge and subscriptionOwner are validated
4. Notice that periodInDays and overridePeriodInSecondsForTestnet are not validated as positive safe integers
5. Try calling subscribe() with invalid periods:

```ts
await subscribe({
recurringCharge: "1.00",
subscriptionOwner,
periodInDays: 0,
testnet: true,
});

await subscribe({
recurringCharge: "1.00",
subscriptionOwner,
periodInDays: -1 as any,
testnet: true,
});

await subscribe({
recurringCharge: "1.00",
subscriptionOwner,
overridePeriodInSecondsForTestnet: 0,
testnet: true,
});
```

### Expected behavior

subscribe() should reject invalid period values before requesting wallet signing.

It should require:
- periodInDays to be a positive safe integer
- overridePeriodInSecondsForTestnet to be a positive safe integer
- the final period value to fit inside the expected uint48 range

### Version

2.5.6 / latest master

### Additional info

Suggested fix: add runtime validation before typed data creation.

Example:

```ts
function validatePositiveSafeInteger(value: number, fieldName: string): void {
if (!Number.isSafeInteger(value) || value <= 0) {
throw new Error(`${fieldName} must be a positive safe integer`);
}
}

function validateUint48(value: number, fieldName: string): void {
const maxUint48 = 2 ** 48 - 1;

if (value > maxUint48) {
throw new Error(`${fieldName} must fit within uint48`);
}
}
```

Then validate before creating typed data:

```ts
validatePositiveSafeInteger(periodInDays, 'periodInDays');
validateUint48(86400 * periodInDays, 'periodInDays');

if (overridePeriodInSecondsForTestnet !== undefined) {
validatePositiveSafeInteger(
overridePeriodInSecondsForTestnet,
'overridePeriodInSecondsForTestnet'
);

validateUint48(
overridePeriodInSecondsForTestnet,
'overridePeriodInSecondsForTestnet'
);
}
```

### Desktop

_No response_

### Smartphone

_No response_

Start in packages/account-sdk/src/interface/payment/subscribe.ts and inspect subscribe() before spend-permission typed data is created. Verify that invalid periodInDays and overridePeriodInSecondsForTestnet values are rejected before wallet signing, and that the resulting period values fit the expected uint48 range.

Written by the indexing model from the issue text.