feat(otp): make OTP email customizable (logo, login URL, button text)

[lane711]

Summary

• Adds admin-configurable branding for the OTP login email so each deployment can match its own product.
• New settings: Logo URL, Logo Width (px), Logo Border Thickness (px), Logo Border Color, Login URL, and Login Button Text.
• Existing emails are visually unchanged when no new settings are provided (border thickness defaults to 0, login button only renders when a URL is set).

Changes

• packages/core/src/plugins/core-plugins/otp-login-plugin/manifest.json — schema + defaults for the new fields.
• packages/core/src/plugins/manifest-registry.ts — mirror defaults in the central plugin registry.
• packages/core/src/plugins/core-plugins/otp-login-plugin/otp-service.ts — extend OTPSettings interface.
• packages/core/src/plugins/core-plugins/otp-login-plugin/email-templates.ts — render configurable logo (width / border / radius) and optional sign-in button; HTML-escape user-provided values; sanitize colors before injecting into inline styles.
• packages/core/src/plugins/core-plugins/otp-login-plugin/index.ts — pass the new settings through to the renderer with safe defaults.
• packages/core/src/templates/pages/admin-plugin-settings.template.ts — admin UI inputs (with a color picker for the border color) and live email preview that reflects each change.

Behavior details

• Logo border only renders when both thickness > 0 and a color is set.
• When the border renders, the logo gets border-radius: 8px to match the sign-in button.
• Default border color is #ffffff; default thickness is 0 (border off).
• Login button label falls back to Sign in to {site name} when blank.
• Plain-text version of the email also gains the CTA URL line when a Login URL is configured.

Testing

• Unit tests pass locally (vitest run src/plugins/core-plugins/otp-login-plugin — 40/40)
• Core typecheck passes (tsc --noEmit -p packages/core/tsconfig.json)
• E2E tests pass in CI

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