UX: Users without accounts hit dead-end on login page

## Problem

Users without accounts are getting stuck in a dead-end when:
1. Trying to sign in on the login page but don't have an account (with self-signup now disabled)
2. Being offered the choice in the FlowFuse expert to continue the conversation from the Website FlowFuse expert to the FlowFuse app

Related: https://github.com/FlowFuse/website/issues/4010#issuecomment-3777111716

## Solution

Add a link to the get-started page (https://flowfuse.com/get-started/) for new users.

Text suggestion:
- "New to FlowFuse?"
- Button: "Try it out"
- "we'll guide you through the next steps"

## Implementation

- [x] Website: Update CTA copy from "Start Free Trial" to "Try it out" - https://github.com/FlowFuse/website/pull/4490
- [x] FlowFuse App: Add "Try it out" button on login page - #6608

This provides a clear onboarding path for users who don't have accounts yet.