feat(release): make the signing-free install work with nothing but Windows

The manual install path still needed 7-Zip, which the target machine does not
have, so "installable without a certificate" was not yet true.

The channel now also publishes the app as byte-split zip parts, and the install
script joins them and extracts with the built-in Windows Expand-Archive after
verifying every part and the joined archive. Version 1.3.1 republishes the
channel from a single build, because a version's artifacts can only match one
build and published volumes are never overwritten.

CHANGELOG.md

@@ -13,6 +13,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Cloud-optional backup (encrypted, opt-in)
 - Plugin system for custom pipelines
 
+## [1.3.1] - 2026-09-18
+
+### Fixed
+- **Manual install no longer needs 7-Zip**: the signing-free channel now also publishes the
+  app as byte-split `zip` parts. The install script joins them and extracts with the built-in
+  Windows `Expand-Archive`, so a user with nothing but Windows can install (the 7z volumes
+  remain the smaller Scoop path). Split volumes from a different build are never mixed:
+  every artifact of a release comes from one build, and a published version is not overwritten.
+
 ## [1.3.0] - 2026-09-18
 
 > Published from an annotated tag through CI. Installer and update metadata are