This PR adresses issue #1735

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## Summary by cubic
Clear chat input and selected component previews when switching apps to
prevent state carryover. Fixes #1735.

- **Bug Fixes**
- Reset chatInputValueAtom and selectedComponentsPreviewAtom on
selectedAppId change in RootLayout.
  - Ensures each app opens with a clean chat and preview state.

<sup>Written for commit df166781e48934ca1e2b8b2b202daa014c358922.
Summary will update automatically on new commits.</sup>

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