docs(account-ui): fix broken framework README links

## Summary
`packages/account-ui/README.md` links to framework-specific docs using paths that do not exist.

## Broken links
- `./react/README.md`
- `./preact/README.md`
- `./vue/README.md`
- `./svelte/README.md`

## What I found
The framework READMEs live under `packages/account-ui/src/frameworks/` instead:
- `src/frameworks/react/README.md`
- `src/frameworks/preact/README.md`
- `src/frameworks/vue/README.md`
- `src/frameworks/svelte/README.md`

## Suggested fix
Update the package README to point to the existing framework documentation paths so GitHub readers can navigate to the correct docs.

Open packages/account-ui/README.md and inspect the four framework links listed in the issue. Compare them with the existing READMEs under packages/account-ui/src/frameworks/{react,preact,vue,svelte}/README.md. Done means all four GitHub links resolve to the corresponding framework documentation.

Written by the indexing model from the issue text.