feat: guide setup through output selection

[hhhjin]

Summary

• require users to choose an output directory before exposing the rest of the TUI setup flow
• move preset output paths into dedicated syncdown subdirectories and require managed output directories to be empty
• reuse the same output directory validation in both TUI and CLI config flows and add coverage for the new behavior

Testing

• bun run lint:fix
• bun test apps/cli/src/program.test.ts packages/core/src/config.test.ts packages/tui/src/*.test.ts

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refines the initial setup experience by introducing a guided output directory selection process. It ensures that users configure a valid and empty output location early in the TUI flow, preventing potential data conflicts. Furthermore, it standardizes preset output paths to dedicated "syncdown" subdirectories and integrates robust directory validation across both command-line and terminal user interface configurations, enhancing overall system stability and user clarity.

Highlights

• User Guidance: The TUI setup now prioritizes output directory selection, guiding users to choose a valid, empty directory before proceeding with other configurations.
• Standardized Output Paths: Preset output paths (Desktop, Documents, Downloads, Home) now default to dedicated "syncdown" subdirectories, ensuring better organization and preventing conflicts.
• Enhanced Validation: A unified validation logic for managed output directories has been implemented and applied across both the TUI and CLI configuration flows, requiring chosen directories to be empty.

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[gemini-code-assist]

Code Review

This pull request effectively implements a guided setup flow by requiring users to select an output directory before proceeding. The new validation ensuring the output directory is empty is a crucial improvement and has been consistently applied to both the CLI and TUI. The related refactoring to centralize this validation logic is well done. The changes are also well-supported by new tests. I have one suggestion to improve code maintainability in the test suite by removing a duplicated helper function.