Keep history imports alive across My Devices recreation

[hamza-lzr]

Follow-up to the rotation finding in #173.

The PR now dismisses the progress window before touching a destroyed Activity, so a completed import no longer tries to show a dialog through a dead window. The blocking subscription is still Activity-owned, however: rotating during a long import loses the visible operation and its eventual result.

Move history-import ownership into a ViewModel so that:

• the import continues across Activity recreation;
• the recreated My Devices screen reconnects to current progress;
• exactly one final success or failure result is delivered;
• added rows still refresh the device list and notify the map;
• leaving the screen intentionally does not leak the Activity.

The progress/importer seam added in #173 should allow this to be tested with a latch-controlled fake.