Port batch image export onto the sras_viewer package split

origin/main split sras_viewer.py into the sras_viewer/ package (dialogs,
main_window, common, canvases) after the batch-export feature was built
against the old monolith, so merging origin/main into main lost the feature
to the sras_viewer.py deletion. Re-add it in the new structure:
BatchExportDialog in dialogs.py, a Jobs.EXPORT job key in common.py, and
the menu action/worker wiring in main_window.py, following the Jobs-key
and dialog-class conventions the split already established rather than the
old ad hoc "main" progress-key naming the original commit used.

Also carries forward two test fixes bundled in the original batch-export
commit that the parallel pytest conversion (007089d) had ported from
tools/test_refactor.py without them: the int16-vs-float32 accumulator
expectation in test_sras_average, and asserting the sras_average.py
subprocess calls actually succeed.

sras_viewer/common.py

@@ -60,6 +60,7 @@ class Jobs:
     COMPUTE = "compute"
     DC_PRECOMPUTE = "dc_precompute"
     BATCH = "batch"
+    EXPORT = "export"
     # The alignment wizard's three background steps: fetching each angle's CH4
     # image for the mask stack, registering the angles, and writing the aligned
     # export. Separate keys because a retry of one must not be blocked by