Split sras_viewer.py into a package (pure move)

The 2790-line module becomes sras_viewer/: common.py (constants +
layout helpers), canvases.py (RoiQuad, ImageCanvas, WaveformCanvas,
ManualAlignOverlayCanvas), dialogs.py (FftOptionsDialog,
ManualAlignmentDialog), main_window.py (SrasViewerWindow + main), with
__init__ re-exporting the public names and __main__ keeping
`python -m sras_viewer` working. pyproject gains a `sras-viewer`
console script.

Code moved verbatim; only import headers are new (pyflakes-clean).
tests/test_gui.py patch targets follow the classes to their new
modules.

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

tests/test_gui.py

@@ -186,7 +186,7 @@ def test_roi_and_csv_export(ctx):
     assert win.btn_export_roi.isEnabled(), "Export ROI enabled"
 
     csv_path = ctx.tmpdir / "roi.csv"
-    with patch("sras_viewer.QFileDialog.getSaveFileName",
+    with patch("sras_viewer.main_window.QFileDialog.getSaveFileName",
                return_value=(str(csv_path), "")):
         win._on_export_roi_csv()
     assert csv_path.exists(), "ROI CSV written"
@@ -195,7 +195,7 @@ def test_roi_and_csv_export(ctx):
         f"ROI CSV has {len(body)} lines for {npix} pixels (want header + one per pixel)"
 
     img_csv = ctx.tmpdir / "img.csv"
-    with patch("sras_viewer.QFileDialog.getSaveFileName",
+    with patch("sras_viewer.main_window.QFileDialog.getSaveFileName",
                return_value=(str(img_csv), "")):
         win._on_export_csv()
     assert img_csv.exists(), "image CSV written"
@@ -426,7 +426,7 @@ def test_stale_schema_sidecar_ignored(ctx):
 
 def test_clear_with_confirmation(ctx):
     win, dlg, s = ctx.win, ctx.dlg, ctx.s
-    with patch("sras_viewer.QMessageBox.question",
+    with patch("sras_viewer.dialogs.QMessageBox.question",
                return_value=QMessageBox.StandardButton.Yes):
         dlg._on_clear()
     assert not ctx.sidecar.exists(), "sidecar file deleted"