Navbar sometimes disappears outside `/onboarding`.
### Changes 🏗️
This PR solves the problem of disappearing Navbar outside `/onboarding`
by introducing `app/(platform)` route group.
- Move all routes requiring Navbar to `app/(platform)`
- Move `<Navbar>` to `app/(platform)/layout.tsx`
- Move `/onboarding` to `app/(no-navbar/`
- Remove pathname injection to header from middleware and stop relying
on it to hide the navbar
### 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] Common routes work properly
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