Add check for missing dependencies on startup

[chennes]

No description provided.

[Copilot]

Pull Request Overview

This PR adds functionality to check for missing dependencies on startup in the Addon Manager. The system now automatically scans installed addons for missing workbenches, external addons, and Python packages, then presents an optional dialog to help users install them.

• Implements a new CheckForMissingDependenciesWorker to scan for missing dependencies during startup
• Adds UI dialog functionality to handle missing dependency installation
• Introduces a preference system to allow users to ignore specific missing dependencies

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
addonmanager_workers_startup.py	Adds new worker class to check for missing dependencies across installed addons
addonmanager_preferences_defaults.json	Adds preference for tracking ignored missing dependencies
addonmanager_installer_gui.py	Moves dependency dialog initialization to constructor for reuse
AddonManager.py	Integrates missing dependency checking into startup sequence and adds post-startup dialog handling
Addon.py	Adds join method to combine multiple MissingDependencies objects

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[github-actions]

Successfully created backport PR for main:

• [Backport main] Add check for missing dependencies on startup #208