[codex] Förbättra updaterloggning

[perNyfelt]

What changed

• Adds explicit Java-side logging for update download, checksum verification, extraction, staging, and updater handoff.
• Adds a persistent updater script log at the app log location (updater.log) and records backup, copy, config update, cleanup, launch, and failure paths.
• Moves update apply/restart work off the Swing completion callback and shows clearer update phases in the dialog, including the updater log path.
• Sets Linux launches to use -Djdk.lang.Process.launchMechanism=VFORK in the packaged launcher and as a guarded startup default.

Why

A 1.2.0 to 1.4.0 in-app update could reach 100% and then appear stuck with no updater log in the installation directory. The app log showed Linux subprocess startup failures (posix_spawn failed, error: 13), which is relevant because the updater relies on launching a detached script after the app shuts down.

Impact and risk

• Users get actionable progress messages and a concrete updater log path when update handoff starts.
• Failed updater scripts should now leave persistent diagnostics under the app log directory instead of disappearing with the detached process.
• The Linux process-launch workaround is limited to Linux and preserves any explicitly provided jdk.lang.Process.launchMechanism value.
• No schema changes.

Validation

• ./gradlew spotlessApply
• ./gradlew :app:compileGroovy codenarcMain test --tests 'unit.se.alipsa.accounting.service.UpdateServiceTest' --tests 'unit.se.alipsa.accounting.support.ResourceBundleCompletenessTest'
• ./gradlew build
• git diff --check