Replace Windows EXE installer with portable bundle for Book Importer

[arieldaniely]

Motivation

• Move the Book Importer distribution from a Windows EXE/MSI installer to a portable ZIP/folder bundle to simplify distribution and allow use without installation.
• Provide a cross-platform-friendly portable artifact that can be copied to USB or network drives and launched directly.
• Keep backward compatibility for existing commands and CI/job names so older scripts still work.

Description

• Update the GitHub Actions workflow name and job from Build Book Importer EXE / build-windows-exe to Build Book Importer Portable / build-windows-portable, change the Gradle task run to packageBookImporterPortable, and update uploaded artifact name and paths to the portable ZIP and app folder at generator/bookimporter/build/compose/binaries/main-release/portable/ and .../app/.
• Add a root Gradle task packageBookImporterPortable that depends on :bookimporter:packagePortable and keep the old packageBookImporterWindowsExe task as a compatibility alias that depends on the portable task.
• Update generator/bookimporter/README.md to document the portable bundle workflow and keep a note about legacy command compatibility using packageBookImporterWindowsExe and :bookimporter:packageWindowsExe as aliases.
• In generator/bookimporter/build.gradle.kts remove the explicit targetFormats installer configuration, add a packagePortableBundle Zip task that packages the app folder into a portable ZIP, add packagePortable to build the portable bundle, and make packageWindowsExe a compatibility alias that depends on packagePortable.

Testing

• No automated tests or CI runs were triggered for this PR; changes are limited to build and packaging tasks and documentation so CI should be run after merging to validate artifact creation on windows-latest.

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