hotfix(frontend): flags copilot redirects (#11878)

## Changes 🏗️

- Refactor homepage redirect logic to always point to `/`
- the `/` route handles whether to redirect to `/copilot` or `/library`
based on flag
- Simplify `useGetFlag` checks
- Add `<FeatureFlagRedirect />` and `<FeatureFlagPage />` wrapper
components
- helpers to do 1 thing or the other, depending on chat enabled/disabled
- avoids boilerplate code, checking flagss and redirects mistakes
(especially around race conditions with LD init )

## Checklist 📋

### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Log in / out of AutoGPT with flag disabled/enabled
  - [x] Sign up to AutoGPT with flag disabled/enabled
  - [x] Redirects to homepage always work `/`
  - [x] Can't access Copilot with disabled flag