Persist FFT settings, fix backend naming, parallelise batch DC caching

- FFT backend and pad factor persist across sessions via QSettings
  (IniFormat; tests redirect the settings path for hermeticity).
- The default backend was labelled "NumPy FFT" but always dispatched to
  scipy.fft — rename the canonical value to "scipy" ("numpy" stays as a
  legacy alias) and fix the dialog label.
- cache_file: DC caching fans out over angles via _parallel_map with
  per-angle budgets (the DcPrecomputeWorker pattern); FFT caching stays
  serial per angle because compute_rf_image now parallelises internally
  over blocks. Documented that the v7 FFT cache is natural-resolution
  (pad 1) by design.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_compute.py

@@ -165,7 +165,7 @@ def test_zoom_identity(tmp_path, monkeypatch, spf, bps):
     dc4 = dc_image_mv(sras, 0, CH4_IDX)
     thr = float(np.median(dc4))
 
-    backends = ["numpy"] + (["pyfftw"] if compute.PYFFTW_AVAILABLE else [])
+    backends = ["scipy"] + (["pyfftw"] if compute.PYFFTW_AVAILABLE else [])
     for backend in backends:
         monkeypatch.setattr(compute, "_fft_backend", backend)
         for pad in (4, 8, 40):