Add Cron Manager extension plugin

[epeicher]

Summary

• Introduces a new top-level extensions/ directory for bundled Desktop Mode extension plugins.
• Adds the first extension: Desktop Mode — Cron Manager (extensions/desktop-mode-cron-manager/), a standalone plugin that registers a native window for browsing, editing, deleting, and running WP-Cron events.
• The plugin declares Requires Plugins: desktop-mode and consumes the documented native-window + <wpd-table> / <wpd-select> / <wpd-text-field> surfaces — no core changes.

Why a separate plugin

Cron management is admin tooling that's useful but not core to the Desktop Mode shell. Shipping it as its own plugin keeps the shell lean, exercises the public registration APIs end-to-end, and gives users an opt-in install path.

Layout

extensions/desktop-mode-cron-manager/
├── desktop-mode-cron-manager.php    plugin bootstrap
├── includes/
│   ├── rest.php                     REST routes for list/run/delete/update
│   ├── store.php                    cron event read/write helpers
│   └── window.php                   native-window registration + bundle delivery
└── assets/
    ├── css/cron-manager.css
    └── js/cron-manager{,.min}.js

Test plan

• Activate desktop-mode and desktop-mode-cron-manager; confirm the Cron Jobs window appears and opens.
• Verify list view renders all scheduled events and timestamps.
• Run an event manually and confirm it executes (and reschedules where applicable).
• Edit an event's next-run timestamp and confirm persistence.
• Delete a one-off event and confirm it's gone after refresh.
• Deactivate the extension; confirm the window unregisters cleanly with no errors.