#3362 [GUI] Reuse Legacy Model Libraries Through A Valid Native Data Root

Beschreibung

## Problem and result

An existing legacy `<Strata Home>/models` library is retained by the host, but passing that directory straight to native setup can fail its new-data-folder name guard. Native preparation now uses `<Strata Home>/Strata-data` with a `models` directory alias pointing to the retained library. Existing weights remain in place.

## Related discussion

Related storage feedback: #3292. This fix addresses the retained legacy HOME/models path specifically; it does not resolve every imported-model recognition problem in that report.

## Approach

Translate only an existing legacy model library, including a saved or environment-selected copy of that exact path. Use a Windows junction or directory symlink for the native layout. Refuse a conflicting destination. Explicit custom libraries and fresh defaults keep their existing behavior.

## Validation

**Passed:**

- Windows, Node 24.18.0, base `e88ee39d`, patch `14c28861`: targeted model-directory checks passed (14 tests in the affected run), including `node --test tests/model-directory-default.test.mjs` from `apps/host`.
- Disposable fixture checks cover detected, saved and environment-selected legacy libraries, idempotence, unchanged original weight bytes, fresh/custom directories and refusal without overwriting a conflicting library. Windows uses a junction in the actual test process.

**Failed:** None remaining in the targeted checks.

**Not run:** Real model preparation/download, GPU generation, macOS/Linux filesystem execution. No engine backend code changed.

## Limits

No model files are moved, copied or downloaded by this fix. Existing arbitrary custom directories remain subject to native setup's existing rules. The alias requires permission to create a directory link at the chosen Strata home.