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>

pyproject.toml

@@ -23,15 +23,18 @@ dependencies = [
 [project.optional-dependencies]
 dev = ["pytest"]
 
+[project.scripts]
+sras-viewer = "sras_viewer.main_window:main"
+
 [tool.setuptools]
 py-modules = [
     "sras_format",
     "sras_compute",
     "sras_workers",
-    "sras_viewer",
     "sras_average",
     "sras_edit_scans",
 ]
+packages = ["sras_viewer"]
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]