[FEATURE] Support Android App Links (or a custom URI scheme) for self-hosted instance domains

Building on #82 (deep links / bookmarkable direct-SSH links) — that feature works great in the browser (?view=terminal&hostId=...), but tapping such a link on Android always opens the system browser instead of the installed Termix app, even when the app is logged into that exact instance. This makes it hard to build notification-based workflows (e.g. via ntfy or other alerting tools) that should deep-link straight into the app instead of the browser.

Two possible approaches:

1. Android App Links per self-hosted domain — since every user's instance domain is different, this would need the app to dynamically verify/register the domain the user configures during setup (rather than a single fixed domain baked into the manifest).
2. A custom URI scheme (e.g. termix://open?view=terminal&hostId=...) — simpler to implement since it doesn't require per-domain Digital Asset Links verification, and could map straight onto the existing fullscreen view params from #82.

Either would make external notification/automation integrations (ntfy, Home Assistant, etc.) much nicer to build, and closes the remaining gap from #82 for mobile users specifically.

Start by reviewing #82 and the existing fullscreen view parameters, then inspect the Android setup for how links are currently handled. Compare the per-domain App Links and custom URI scheme options described here before choosing an approach. Done means an external link can open the installed Termix app and reach the requested self-hosted instance view.