The workspace add endpoint used resolve_trusted_workspace() which blocks any path
outside the user's home directory, the saved workspace list, or BOOT_DEFAULT_WORKSPACE.
This created a circular dependency: to add /mnt/d/Projects you need it in the saved
list, but to get it in the list you need to add it.
Fix: introduce validate_workspace_to_add() used by /api/workspaces/add, which only
blocks non-existent paths, non-directories, and known system roots. The stricter
resolve_trusted_workspace() is still used for actual file operations within a workspace.
Fixes#953.
Co-authored-by: nesquena-hermes <nesquena-hermes@users.noreply.github.com>
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