Extract the embedded Windows installer application into a testable module

[eimexdev]

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.

[eimexdev]

This refactor is useful, but it sits beyond the destination of the OpenBeat v0.1.0 public launch map. Wayfinder rules it out of this effort so it cannot appear on the launch frontier. Reopen it as part of a fresh post-launch maintenance effort if the need remains after release.