packaging: portable data root (QUERYA_PORTABLE / QueryaData)

[ZhuchkaTriplesix]

Parent

Part of #379

Problem

Portable zip runs without install, but SQLite/themes/extensions/logs still use OS app-support paths. True USB-style portable needs an opt-in data root next to the binary.

Proposal

1. Detect portable mode when:
   • env QUERYA_PORTABLE=1, or
   • sidecar directory QueryaData/ next to querya_desktop / querya_desktop.exe / $APPIMAGE
2. Centralize path resolution (DB, themes, extensions, logs) behind one root helper; OS paths remain default for installable builds.
3. Document that secrets still use OS keyring in v1 (libsecret / Credential Manager / Keychain) unless a later issue adds a portable secrets store.

Acceptance

• Single resolver used by local_db, theme paths, extension paths, sandbox logs
• Unit/integration tests for detection + path redirect
• Docs: how to enable portable profile; what stays in the OS keyring
• Installable / normal builds unchanged when flag/folder absent

References

• lib/core/storage/local_db.dart
• lib/core/theme/theme_paths.dart
• lib/core/extensions/extension_paths.dart
• lib/core/extensions/sandbox/sandbox_log_paths.dart