Auto-redirect logged-in users from flowfuse.com to app.flowfuse.com

## Problem

Currently, existing users who navigate to `flowfuse.com` must manually click the "Sign In" button to get to `app.flowfuse.com`, creating unnecessary friction in the user journey.

Quite a number of unique visitors clicked "Sign In" from the homepage in a 7-day period (late October 2025), suggesting these users just wanted to access their environment, not view marketing content.

## Current Behavior

- User types `flowfuse.com` or clicks a bookmark
- User sees marketing homepage
- User manually clicks "Sign In"
- User is redirected to `app.flowfuse.com`

## Desired Behavior

Similar to GitHub's approach:
- User navigates to `flowfuse.com`
- JavaScript detects if user is logged in (via API call to `app.flowfuse.com`)
- If logged in, automatically redirect to `app.flowfuse.com`
- If not logged in, show marketing homepage as normal

## Proposed Solutions

### Short-term: Homepage Auto-redirect
Add JavaScript to the homepage that:
1. Checks if the user is logged in by making an API call to `app.flowfuse.com`
2. Redirects logged-in users to `app.flowfuse.com`
3. Shows marketing content to non-logged-in users

**Technical Feasibility**: Confirmed feasible by @knolleary in Slack discussion (Oct 28, 2025)

### Long-term: Subdomain Strategy (Optional)
Consider adopting GitLab's approach with an `about.flowfuse.com` subdomain:
- Move marketing content to `about.flowfuse.com`
- `flowfuse.com` becomes the app entry point
- Provides cleaner separation between marketing and product
- Makes docs/pricing accessible without losing app access

## Considerations

1. **Navigation from App**: If homepage redirects logged-in users, we need better links from the app back to:
- Documentation
- Pricing information
- Other website resources

2. **Performance**: API call to check login status should be fast and not delay page load significantly

3. **Privacy**: CORS and cookie policies need to support cross-subdomain authentication checks

## Examples

- **GitHub**: Detects logged-in users on homepage and redirects to app
- **GitLab**: Uses `about.gitlab.com` for marketing, `gitlab.com` for the app

## Related Discussion

Original Slack thread in #design (Oct 28, 2025): https://flowfuse.slack.com/archives/C03387T7UFP/p1761662521430019

## Acceptance Criteria

- [ ] Logged-in users navigating to `flowfuse.com` homepage are automatically redirected to `app.flowfuse.com`
- [ ] Non-logged-in users continue to see marketing homepage
- [ ] Login detection does not significantly impact page load time
- [ ] Solution works across all major browsers
- [ ] Analytics track how many users are auto-redirected vs. manually signing in