feat(packaging): make portable mode out-of-the-box via .portable marker (#1056)

[ZhuchkaTriplesix]

Closes #1056

Description

Enables zero-configuration portable mode for official release zip archives (Windows & Linux).
Previously, portable mode required manually creating a QueryaData folder or passing QUERYA_PORTABLE=1. Without this, portable zips silently stored user databases and configurations in %APPDATA%\querya_desktop or ~/.local/share/com.queryahub.querya_desktop.

Changes

1. AppDataRoot (lib/core/storage/app_data_root.dart):
   • Added support for marker files (.portable and portable.txt).
   • If marker is present, automatically creates and redirects to QueryaData/ next to the application binary on launch.
2. Release Workflow (.github/workflows/release.yml):
   • Injects .portable and portable.txt markers into Windows and Linux portable release zip archives.
   • Ensures installer packages (setup.exe, .deb, .rpm, Flatpak) remain clean and installable to system locations.
3. Preferences About Section (lib/features/settings/preferences_about_section.dart):
   • Added a clear portable mode status indicator using QueryaBadge.status('PORTABLE', status: QueryaBadgeStatus.info) explaining where data and passwords are kept.
4. Docs & Tests:
   • Updated docs/packaging.md.
   • Added unit tests in test/core/storage/app_data_root_test.dart verifying marker detection and automatic directory creation.