Promote Unreleased to v1.5.0

Auto-update infrastructure is now ready to ship. v1.5.0 introduces the Setup.exe installer + in-app auto-update banner + Settings dialog Updates section + Velopack-backed delta packages for small updates. Existing portable users see no behavior change beyond a new download option on the Releases page.

AI-Local-Session: 4519f6b6-393a-4476-8efa-410e5396c3a9

AI-Cloud-Session: 72f9e474-60ab-42c2-b2a0-28fee827cbbb

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Geeven Singh

CHANGELOG.md

@@ -12,6 +12,8 @@ body. Keep section headings exact and write notes in Markdown.
 
 ## [Unreleased]
 
+## [1.5.0] - 2026-05-30
+
 ### Added
 
 - **`DiffViewer-Setup.exe` installer alongside the existing portable