Only expose __REDUX_STORE__ in test mode

[legoktm]

We only need it there, so this reduces our attack surface slightly. Flagged by Claude 5 Opus.

Test plan

• visual review is sufficient, just grep that it's not used anywhere outside the server test

Checklist

This change accounts for:

• testing changes on Qubes as needed (especially changes related to cryptography, export, disposable VM use, or complex UI changes)
• any needed updates to the AppArmor profile for files beyond the application code
• any needed self-contained database migrations (including testing against a clean test database from main)

[legoktm]

We don't need the typeof window check anymore (or ever really) because the very next line invokes window.electronAPI...