fix: export desktop to a invalid export path

[dengbo11]

1. The custom desktop exported directory(CUSTOM_DESKTOP_DATA_DIR) should only be in the share dir, not in the subdirectory;
2. LINGLONG_DATA_DIR should be exported;
3. need to update LINGLONG_EXPORT_VERSION to fix export.

[Copilot]

Pull Request Overview

This PR refactors how XDG_DATA_DIRS paths are normalized and added—introducing a helper function—and bumps the export version.

• Normalize and sanitize LINGLONG_DESKTOP_EXPORT_PATH, LINGLONG_DATA_DIR, and CUSTOM_DESKTOP_DATA_DIR
• Introduce _add_path_to_xdg_data_dirs helper to safely prepend or append paths
• Ensure LINGLONG_DATA_DIR is always exported and bump LINGLONG_EXPORT_VERSION to 1.0.0.2

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
misc/lib/linglong/generate-xdg-data-dirs.sh	Add path-normalization logic and helper function
libs/utils/src/linglong/utils/configure.h.in	Update LINGLONG_EXPORT_VERSION from 1.0.0.1 to 1.0.0.2

Comments suppressed due to low confidence (2)

misc/lib/linglong/generate-xdg-data-dirs.sh:11

• This comment refers only to appending, but the new helper supports both prepend and append. Consider updating it to explain the helper’s full capabilities.

# instead of prepending it like flatpak.

misc/lib/linglong/generate-xdg-data-dirs.sh:55

• [nitpick] The closing fi is indented by two spaces, which is inconsistent with other if…fi blocks in this script. Align it with its if for clearer structure.

  fi