[PierreRaybaut]

Summary

Closes #419.

Companion public catalog PR: DataLab-Platform/plugins#4.

Allow DataLab Desktop's online and offline plugin catalogs to distribute unchanged single-file datalab_*.py plugins alongside wheels. Packaging a script as a project is no longer a prerequisite for catalog distribution.

This branch starts directly from develop, with no experimental-data-integration commits. The first commit clarifies source organization, distribution and functional capabilities before any code change; the final documentation commit describes the implemented workflows in English and French.

Changes

• Add a Qt-free, non-executing module inspector, shared with the public catalog. Read only literal metadata and simple constant references; dynamic or ambiguous metadata remains unknown and can be supplied by a catalog.
• Extend catalog records compatibly with kind: module and the real module name, without inventing a Python distribution. Existing wheel entries retain their default behavior.
• Support mixed root folders and authoritative JSON indexes, including local catalog build/check/prepare commands and mirrored .py downloads with size/SHA-256 checks.
• Track unversioned modules by digest. Recognize previous file installations, display identical content as installed, update known versions, and request explicit replacement for changed revisions.
• Reuse managed module installation, hot reload and uninstall; preserve old index entries and refuse collisions with unmanaged modules or conflicting managed identities.
• Verify the accepted file again at installation and restore the previous module if updating the index fails.
• Preserve wheel dependency management. Single-file modules use existing host libraries and never invoke pip or install a dependency wheelhouse.
• Extend the frozen-build probe to install and load both a historical unversioned script and a generated single-file plugin through a local catalog.
• Complete the independent source-form/distribution matrix, offline procedures, author guidance, French UI strings and release notes.

Validation

• Full Desktop plugin regression run: 208 passed across datalab/tests/backbone/plugins and datalab/tests/features/plugins/plugin_install_dialog_test.py.
• Final focused module/catalog run after adding the explicit-version guard: 34 passed.
• Public catalog companion tests, including consumption of its published mirror by this Desktop client and legacy-host detection: 38 passed.
• Ruff checks and formatting passed; 1755 French UI messages compiled. All UI strings in the affected workflows were checked against the translation catalog.
• Sphinx gettext and French source/reference validation passed with -W. The plugin chapter's 529 messages and the release notes' 136 messages are translated without missing/fuzzy entries.
• Installed unchanged historical and generated scripts from a local catalog in the Python edition and a real Windows CPython 3.11 PyInstaller standalone. Neither ran during inspection or installation; both ran from managed storage at application startup. The standalone used no external Python.
• Executed the original origin/develop catalog client on a mixed catalog: it skipped the module and retained the existing installable wheel.
• Covered invalid syntax/encoding/names, empty and oversized files, metadata contradictions, digest changes after consent, cancellation, source priority/withdrawals, name collisions and failed-index restoration.

PDF generation was intentionally not repeated: the maintainer confirmed that validating the documentation sources is sufficient for the established integrated-help build.

Scope

Desktop only, including its standalone distribution. No application DataLab-Web changes, extra dependency installation for scripts, release publication or production plugin entry is included. The public catalog schema, release-asset validation, mirror and website changes are delivered in the linked companion PR.