Improve Add space with folder selection and recent folders

Improve the existing Add space dialog with a desktop folder picker, editable directory and recent-folder selection. Blank or whitespace-only names use the selected folder's name, with a sensible filesystem-root fallback; explicit names remain untouched. Remember at most 12 successfully added canonical directories, most recent first and deduplicated, even after removing a space. Persist history with the workspace layout using existing bounded transactional decoding and private atomic storage; temporary workspaces stay isolated. Existing layouts remain readable and seed history from their spaces. Cancellation or invalid input must not add history or launch processes. Verified: build succeeds; all 7 CTest suites and all 21 private Wayland/OpenGL UI test cases pass. Coverage includes naming/root/whitespace/tilde and special-character URL handling, canonical deduplication and history bounds, removed-space persistence and private file permissions, legacy layouts and atomic rejection, inline invalid-input handling, recent-folder mouse selection, actual fallback-picker folder selection/acceptance/cancellation, Enter submission, and dialog bounds at 840×520 and 1360×520. Screenshots visually inspected. The picker test uses Qt's fallback dialog; desktop-specific native/portal implementations were not exercised.