feat(metrics): mark desktop installation source

[Soulter]

Summary

• Set ASTRBOT_INSTALLATION_SOURCE=desktop when launching the packaged AstrBot backend.
• Extend the desktop-managed command test to cover the installation source marker.
• Document the new desktop environment variable.

Why

AstrBot's anonymous metrics need a stable installation-source marker so desktop launches can be distinguished from other installation methods.

Impact

Packaged desktop launches now identify their installation source as desktop; existing desktop management markers remain unchanged.

Validation

• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check
• cargo test --manifest-path src-tauri/Cargo.toml mark_as_desktop_managed_sets_desktop_management_markers

Summary by Sourcery

Mark packaged desktop backend launches with a stable installation-source environment variable for metrics.

New Features:

• Introduce the ASTRBOT_INSTALLATION_SOURCE=desktop environment variable for packaged desktop backend launches.

Documentation:

• Document the new ASTRBOT_INSTALLATION_SOURCE desktop environment variable in the environment variables reference.

Tests:

• Extend the desktop-managed command test to assert the installation source environment variable is set for packaged desktop launches.

[sourcery-ai]

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider whether mark_as_desktop_managed should respect a pre-existing ASTRBOT_INSTALLATION_SOURCE env value (e.g., only set it if not already present) to allow explicit overrides in more complex launch scenarios.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider whether `mark_as_desktop_managed` should respect a pre-existing `ASTRBOT_INSTALLATION_SOURCE` env value (e.g., only set it if not already present) to allow explicit overrides in more complex launch scenarios.

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