Login button reads "Sign In" while the rest of the card uses sentence case

## What

The submit button on the login form says "Sign In" with a capital I. Every other occurrence of the phrase on that same screen uses sentence case.

## Where

`src/app/login/login-form.tsx` line 376:

```tsx
{isLoading ? "Authenticating..." : mfaRequired ? "Verify code" : "Sign In"}
```

The same file, a few lines up:

- line 239: `Sign in`
- line 242: `Sign in to your LibreDB Studio account`
- line 263: `Sign in securely with your organization's identity provider`

The button is the odd one out, and the MFA branch beside it already reads `Verify code` in sentence case.

## Expected

The button reads `Sign in`, matching the rest of the card.

## How to fix

1. In `src/app/login/login-form.tsx` line 376, change `"Sign In"` to `"Sign in"`.
2. Check `e2e/login.spec.ts` for a selector that matches the button by its text. If one does, update it in the same pull request so the suite still passes.

## How to check

Run `bun run dev`, open `/login`, and read the button. Then run the login end-to-end spec and confirm it still passes.

Open src/app/login/login-form.tsx at the submit-button text around line 376 and compare it with the surrounding sentence-case labels. Check e2e/login.spec.ts for a text-based selector, run the login end-to-end spec, and verify that the /login button reads “Sign in” while the test still passes.

Written by the indexing model from the issue text.