refactor(bookimporter): switch to portable distribution workflow

[arieldaniely]

Motivation

• Make the Book Importer distribution lightweight and portable for one-off usage rather than producing OS installers.
• Simplify distribution and avoid installer/platform-specific packaging complexity.

Description

• Replace installer-oriented packaging in generator/bookimporter/build.gradle.kts by removing installer targetFormats and adding a packagePortable task that depends on createReleaseDistributable to produce a portable app folder.
• Update root helper task in build.gradle.kts to packageBookImporterPortable which depends on :bookimporter:packagePortable.
• Revise generator/bookimporter/README.md to document portable usage (copy/move the generated app folder and run the launcher) and update the example Gradle commands to :bookimporter:packagePortable / packageBookImporterPortable.
• No schema or public API changes; change is restricted to build/packaging and docs.

Testing

• Ran ./gradlew :bookimporter:tasks --all | rg -n "packagePortable|createReleaseDistributable" to verify the new module task is registered and wired to createReleaseDistributable (succeeded).
• Ran ./gradlew tasks --all | rg -n "packageBookImporterPortable" to verify the new root helper task (succeeded).
• Ran ./gradlew :bookimporter:packagePortable to build the portable distribution and verified the distributable folder is created at generator/bookimporter/build/compose/binaries/main-release/app (build completed successfully).
• An initial script compilation error related to installer TargetFormat usage was observed during iteration and resolved by switching to createReleaseDistributable before final verification (fixed, final build succeeded).

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