Extract the embedded Windows installer application into a testable module #25

Part of #9.

Context

scripts/build_installers.py currently embeds the Windows installer application as a large generated Python string. Packaging, UI, process detection, path handling, and installation logic therefore change in one builder file and most installer behavior cannot be imported directly by tests.

Work

• Move the Windows installer application into a normal source module.
• Give install results a typed contract rather than a string-keyed dictionary.
• Share Windows support-path construction instead of duplicating APPDATA handling.
• Add focused tests for path selection, copying, configuration generation, upgrade replacement, and failure reporting.
• Keep the produced installer behavior and artifact naming stable.

Acceptance criteria

The installer application is directly importable/testable, build orchestration is smaller, and the current release behavior remains intact.

This is marked post-launch unless smoke testing reveals a blocking installer defect.