Fix packaged asset resolution for issue #78

[thisislvca]

Summary

• restore the packaged asset root fallback for Windows/Linux frozen builds so resources resolve from _internal/
• build device image URLs with QUrl.fromLocalFile(...) instead of concatenating file:/// strings in QML
• pass the app root into the backend so image asset resolution comes from a single source of truth
• add a regression test covering a root path that contains spaces

Why

Issue #78 looks like the HID++ device-identification fix and the blank schema are separate problems. The screenshots show the device is identified correctly, but the mouse image never loads in the packaged app, which collapses the hotspots into the center.

A stronger suspect is packaged resource resolution: older frozen builds explicitly resolved assets from _internal/, while current code could fall back to the executable directory instead. This keeps the packaged fallback and also tightens the image URL handling in QML.

Validation

• .venv/bin/python -m unittest tests.test_backend -q
• manual sanity check of main_qml._resolve_root_dir() for dev, frozen fallback, and _MEIPASS cases

Refs #78