Rename to Sendspin for Windows

[chrisuthe]

Rebrands the app from WindowsSpin / SendspinClient to Sendspin for Windows (display) with code identity Sendspin.Windows, in preparation for moving the repo into the Sendspin org. The repo transfer and all GitHub URL rewrites are deferred — URLs still point at chrisuthe/windowsSpin so nothing breaks during validation.

What changed

Layer	Result
Display name	WindowsSpin → Sendspin for Windows (window title, toasts, product metadata, logs, dialogs)
Code identity	SendspinClient → Sendspin.Windows (namespaces, projects, solution, Sendspin.Windows.exe)
AppData folder	%LocalAppData%\WindowsSpin → %LocalAppData%\Sendspin for Windows with a one-time, idempotent migration that preserves client_id (server pairing survives upgrade)
Installer	New exe/output names; AppId GUID kept for in-place upgrade; [InstallDelete] removes the orphaned SendspinClient.exe and old shortcuts; post-install script defers to the app's migration
CI + docs	Workflow project/artifact paths, CLAUDE.md (incl. a stale \Sendspin\ AppData path fix), README, CONTRIBUTING

Upgrade behavior for existing installs

• Settings, logs, and the client_id are migrated on first launch — existing users keep their configuration and stay paired.
• Because the AppId is preserved, an in-place upgrade keeps the old install-dir / Start-Menu-group folder names (shortcut labels and files do update). Fresh installs use "Sendspin for Windows".

Verification

• dotnet build -c Release Sendspin.Windows.sln → 0 errors.
• Tests: 97 pass, 2 fail — both failures are pre-existing DSP/sync tests (resampler SUT is byte-identical to master bar namespace lines; the tests also fail on master), not caused by this rename.
• AppData migration verified end-to-end (legacy folder moved, client_id preserved).

Deferred (separate follow-up when moving to the org)

• Repo transfer to Sendspin/sendspin-windows, GitHub URL rewrites, origin remote change, and the winget publisher identifier. Tracked in docs/superpowers/plans/2026-07-13-rename-sendspin-for-windows.md (Appendix A).

Known cosmetic non-blockers

• Serilog log-file prefix is still windowsspin-.log (internal filename only).
• Some historical design docs under docs/plans/ still cite old paths (out of scope for this rename).