topnav: add login modal

[ericholscher]

Adds a #login-modal in topnav.html mirroring the existing sign-up modal, giving Community and Business options that link to the respective dashboards. The modal opens via the #login hash, which showModalsByHash() already maps. The topnav Log in dropdown and Sign up button are unchanged.

Copy is written for returning users (not decision-making for new users): intro "Log in to the site where your account is registered", one-line platform reminders, and actions "Not sure which site?" / "Don't have an account? Sign up".

Screenshot

Screenshot is committed under .github/screenshots/ for review. Remove before merge if preferred. (Font Awesome Pro icons don't render in the local build, so the card headers appear text-only here; they'll appear with icons in production.)

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Generated by AI

[humitos]

Why this PR is not getting the comment from the app? I was looking the link to click and test it. Don't we have it configured on this repository?

[humitos]

It looks like this 👍🏼

[humitos]

By the way, I would change the "Login" link to show this modal instead of the dropdown to unify everything. When clicking this link it should show the modal.

[ericholscher]

Yea, I didn't update the link to just try it out as a link from the docs first. Wasn't sure the right approach.

[humitos]

It would be good to move forward with this PR so we standardize these buttons 👍🏼

[ericholscher]

Honestly I'd like to fully remove the modals and just link to specific pages I think. Modals are a pretty bad UX in my experience, especially with these with hash issues, I'd love to just be able to build these as normal pages that can be linked to like normal :)