feat(artifacts): move files within workspace folders

[juacker]

Summary

• add a guarded workspace_move_path command for same-workspace artifact moves into folders
• make artifact folder rows accept same-workspace move drops and expand after 2 seconds of drag hover
• use atomic no-overwrite rename primitives, including Windows MoveFileExW(..., 0), so collision handling cannot replace existing files

Verification

• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• npm run typecheck
• npm run lint
• cargo test --manifest-path src-tauri/Cargo.toml commands::workspace::tests -- --nocapture
• git diff --check

Review

• Codex bounded final review: production_quality
• Minimax follow-up final review: production_quality
• Claude final review after login fix/restart: production_quality