packaging: updater selects asset by install context (zip vs AppImage vs setup)

[ZhuchkaTriplesix]

Parent

Part of #379

Problem

platformAssetFor (and related selection) only looks for *-{linux,windows,macos}.zip. Once Releases publish AppImage / setup.exe, portable users must not be force-switched to installers and vice versa.

Proposal

• Detect context: APPIMAGE, Windows installed-vs-portable heuristic, macOS .app location.
• Prefer matching artifact family; fall back documented if missing.
• Keep Snap/Flatpak short-circuit (package manager update) as today.

Acceptance

• Unit tests for asset preference matrix
• Manual matrix: portable zip / AppImage / setup.exe each pulls the right file
• Clear user-facing error if preferred asset missing from Release

References

• lib/core/updater/app_updater_service.dart
• lib/core/updater/update_platform_installer.dart
• lib/core/updater/installers/update_install_context.dart