Add backend-only installation mode across platforms

[bhavsarpratik]

Summary

Adds a backend-only installation mode for macOS, Linux, and Windows. It installs and starts the local Token Meter server while skipping native menu-bar/tray companions, removes an existing owned companion when switching modes, and preserves the selected mode through startup and automatic updates.

Without the new flag, existing full installations continue to install the server and native companion.

Validation

• Focused installer, packaging, and update coverage: 43 passed; 3 expected platform skips.
• Shell syntax, Python compilation, runtime-manifest validation, and git diff --check passed.
• Exact-head requirements and platform reviews found no findings.
• The macOS full runtime was restored and verified after testing.

Limitations

Native Windows and Linux runtime behavior has not been exercised on target hosts; their behavior is covered by source and packaging contracts. The full suite also retains two unrelated pre-existing failures: the ignored docs/.DS_Store inventory check and an existing README Pi wording expectation.