As a Proton Pass user who works across many machines, some of them offline, locked down, or not my own, I want a portable version of Proton Pass that runs from a USB drive or other removable device, with my encrypted vault stored locally on it, so that I can reach my passwords securely without installing software, signing in through a browser, or relying on an internet connection.

Context:
I regularly work on computers where I can’t install apps, such as work PCs, client machines, and air-gapped or restricted systems. I also work in places with poor or no connectivity. Today, Proton Pass assumes either an installed client or an online session, so in these situations my passwords are effectively locked away. Many people in this position end up using a second tool like KeePassXC, which splits their vault and defeats the point of having one trusted password manager.

Acceptance criteria:
* A self-contained build for Windows, macOS, and Linux that runs without installation or admin rights
* The vault is stored end-to-end encrypted on the portable device and unlocks offline with my password, PIN, and/or a hardware key
* Leaves no data behind on the host machine (no cache, logs, or registry entries)
* Syncs with my Proton account whenever a connection is available, with clear handling of conflicts
* Optional read-only mode for extra safety on untrusted machines