Fix local development console errors

[areibman]

📥 Pull Request

📘 Description
This PR addresses the excessive console errors in local development mode by:

• Gating PostHog initialization to only run in production or when explicitly enabled via NEXT_PUBLIC_ENABLE_POSTHOG_IN_DEV=true.
• Adding checks to signin and simple-login pages to prevent unnecessary /opsboard/users/me API calls (and subsequent 401 errors) when no session_id cookie is present.
• Removing an unused PostHog import.

These changes significantly reduce console noise, improving the local development experience.

🧪 Testing

1. Without logging in: Hard refresh /signin or /simple-login. Verify that the console is clean, with no PostHog 401/404 errors or /opsboard/users/me 401 errors.
2. After logging in: Log in as a user. Verify that the application functions normally and that relevant requests (including /opsboard/users/me) are made without the previous noise.
3. (Optional) Enable PostHog in dev: Set NEXT_PUBLIC_ENABLE_POSTHOG_IN_DEV=true in .env.local and provide PostHog keys. Verify PostHog initializes and sends events in dev mode.

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[cursor]

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