feat: migrate Windows build scripts from client-building to PowerShell in admin/win

[Copilot]

Ports the Windows build pipeline from the external nextcloud/client-building batch scripts into this repository as PowerShell, living in admin/win/build/. Also converts admin/win/msi/make-msi.bat.in to PowerShell.

Changes

admin/win/build/ — new PowerShell build pipeline

File	Replaces	Role
common.ps1	common.inc.bat / datetime.inc.bat	Dot-sourced library: env helpers, Invoke-NativeCommand, Invoke-VsDevShell, Invoke-Sign, Invoke-Upload, Write-BuildStep
defaults.ps1	defaults.inc.bat	Dot-sourced defaults; each variable is a no-op if already set in the environment
init.ps1	init.bat	One-time workspace clone + directory creation
build.ps1	build.bat	Orchestrator: desktop → collect → MSI
build-desktop.ps1	build-desktop.bat + single-build-desktop.bat	CMake configure/build/install + windeployqt, iterates over BUILD_TARGETS
build-installer-collect.ps1	build-installer-collect.bat + single variant	File staging, Qt/OpenSSL/VC Redist copy, optional binary signing
build-installer-msi.ps1	build-installer-msi.bat + single variant	Calls make-msi.ps1, signs + moves + uploads the MSI
sign.ps1	sign.bat	Standalone entry point for Invoke-Sign
upload.ps1	upload.bat	Standalone entry point for Invoke-Upload
task-build-log.ps1	task-build-log.bat	Task Scheduler wrapper using Start-Transcript; exits with build exit code
README.md	(new)	Prerequisites, env-var reference, signing/upload setup, scheduled-build guide, migration table

NSIS installer workflow is intentionally not carried forward; MSI is the sole Windows installer format.

admin/win/msi/make-msi.ps1.in — new

Direct PowerShell port of make-msi.bat.in. Replaces !LF! / setlocal EnableDelayedExpansion tricks with clean loops and [System.Guid]::NewGuid() instead of uuidgen.

admin/win/msi/CMakeLists.txt — updated

Switches configure_file target from make-msi.bat.in → make-msi.ps1.in and updates the install(FILES …) list accordingly.

Notes

• All environment variable names are unchanged — no CI variable config changes needed.
• TEST_RUN=1 dry-run mode is preserved across all scripts.
• No GitHub workflow files are modified; this is a local/scheduled-build change.