Feature: Add simple sign-up page

[mjkatgithub]

Goal

Allow new users to create a Matrix account from the app with a minimal,
homeserver-based sign-up flow.

Scope

• Add a new /signup page as MVP.
• Reuse the current sign-in form structure (homeserver, username, password).
• Add a Sign up button next to Sign in on the landing page (/).
• Keep homeserver editable, prefilled with https://matrix.org.
• Show clear user-facing error messages when sign-up is not available.
• Do not implement homeserver catalog/dropdown in this slice.

Branch

feature/simple-signup-page (Feature)

Acceptance Criteria

• A new Sign up page exists and is reachable via /signup.
• Landing page shows Sign in and Sign up buttons side by side.
• Sign-up form fields match MVP: homeserver URL, username, password.
• Submitting the form attempts Matrix registration on selected homeserver.
• If registration is disabled/unsupported, user gets a clear hint.
• Existing sign-in flow remains unchanged and working.

Test Checklist

Unit

• Add/adjust tests for sign-up page form behavior and error handling.
• Add/adjust tests for landing page auth buttons navigation.

Integration

• Verify successful registration flow against local test homeserver.
• Verify disabled-registration flow returns clear UI message.

E2E

• User can navigate from / to /signup.
• User can register (where allowed) and continue to app/login flow.

Notes

• Priority: prio1.
• MVP-first approach: copy sign-in UX where possible.
• Future iteration: homeserver presets/dropdown + capability display.