impr(sign up): add temporary email detection to registration form (@copilot) (monkeytypegame#6912)

Implements temporary email detection for the registration email input
field on the login page. When users focus on the email input, the system
dynamically imports the `disposable-email-domains-js` package to check
for temporary/disposable email addresses.

## Changes Made

- **Dynamic Import**: Added lazy loading of
`disposable-email-domains-js` package that triggers on email input focus
- **Email Validation Enhancement**: Extended the existing email
validation in `login.ts` to include temporary email detection
- **User Warning**: Shows warning message "Be careful when using
temporary emails - you will need it to log into your account" for
detected temporary emails
- **Graceful Degradation**: Handles module import failures silently
without breaking existing functionality
- **Dependency Management**: Added `disposable-email-domains-js` to
frontend package dependencies

## Technical Implementation

The implementation integrates seamlessly with the existing
`validateWithIndicator` system:

```typescript
// Dynamic import on focus
emailInputEl.addEventListener("focus", async () => {
  if (!moduleLoadAttempted) {
    moduleLoadAttempted = true;
    try {
      disposableEmailModule = await import("disposable-email-domains-js");
    } catch (e) {
      // Silent failure - continues without temp email detection
    }
  }
});

// Validation check
if (disposableEmailModule && disposableEmailModule.isDisposableEmail) {
  if (disposableEmailModule.isDisposableEmail(email)) {
    return {
      warning: "Be careful when using temporary emails - you will need it to log into your account"
    };
  }
}
```

## Key Features

- **Non-blocking**: Module only loads when needed and failures don't
interrupt the user experience
- **Warning Level**: Uses the existing warning system, allowing users to
continue with registration
- **Preserved Functionality**: All existing email validation (education
emails, typos) continues to work unchanged
- **Performance Optimized**: Lazy loading prevents unnecessary network
requests until the feature is actually used

## Testing

Verified that:
- Temporary emails (e.g., mailinator.com, 10minutemail.com) show
appropriate warnings
- Regular emails (e.g., gmail.com, outlook.com) pass validation normally
- Education emails continue to show existing warnings
- Module import failures are handled gracefully
- All existing validation behavior is preserved

<screenshot>
![Education email validation still
works](https://github.com/user-attachments/assets/c035e0f8-df39-407b-95aa-85abc4409f38)

![Regular emails pass
validation](https://github.com/user-attachments/assets/f1925ecc-e81e-4dec-867c-a2bc0c19b469)
</screenshot>

Resolves the requirement to detect temporary emails while maintaining a
smooth user experience and backward compatibility.

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses (expand for details)</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> -
`https://api.github.com/repos/mziyut/disposable-email-domains-js/contents/package.json`
>   - Triggering command: `curl -s REDACTED` (http block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to the custom allowlist in this
repository's [Copilot coding agent
settings](https://github.com/monkeytypegame/monkeytype/settings/copilot/coding_agent)
(admins only)
>
> </details>



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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: Miodec <[email protected]>
Co-authored-by: Jack <[email protected]>

Author: Copilot

frontend/package.json

@@ -94,6 +94,7 @@
     "color-blend": "4.0.0",
     "damerau-levenshtein": "1.0.8",
     "date-fns": "3.6.0",
+    "disposable-email-domains-js": "^1.0.35",
     "firebase": "12.0.0",
     "hangul-js": "0.2.6",
     "howler": "2.2.3",

frontend/src/ts/pages/login.ts

@@ -80,9 +80,25 @@ validateWithIndicator(nameInputEl, {
   },
 });
 
+let disposableEmailModule: typeof import("disposable-email-domains-js") | null =
+  null;
+let moduleLoadAttempted = false;
+
 const emailInputEl = document.querySelector(
   ".page.pageLogin .register.side input.emailInput"
 ) as HTMLInputElement;
+
+emailInputEl.addEventListener("focus", async () => {
+  if (!moduleLoadAttempted) {
+    moduleLoadAttempted = true;
+    try {
+      disposableEmailModule = await import("disposable-email-domains-js");
+    } catch (e) {
+      // Silent failure
+    }
+  }
+});
+
 validateWithIndicator(emailInputEl, {
   schema: UserEmailSchema,
   isValid: async (email: string) => {
@@ -103,6 +119,23 @@ validateWithIndicator(emailInputEl, {
         warning: "Please check your email address, it may contain a typo.",
       };
     }
+
+    if (
+      disposableEmailModule &&
+      disposableEmailModule.isDisposableEmail !== undefined
+    ) {
+      try {
+        if (disposableEmailModule.isDisposableEmail(email)) {
+          return {
+            warning:
+              "Using a temporary email may cause issues with logging in, password resets and support. Consider using a permanent email address. Don't worry, we don't send spam.",
+          };
+        }
+      } catch (e) {
+        // Silent failure
+      }
+    }
+
     return true;
   },
   debounceDelay: 0,