Add project --refit to re-propose the content rectangle

Saved state always wins over a fresh proposal, which is what the
project file is for — but it also means a project made before detection
proposed a content rectangle keeps the old whole-page one forever, and
reopening or re-exporting changes nothing.

--refit re-runs the proposal over every page while leaving cuts,
discards, stepped cuts and metadata untouched. Verified on the real
Engel project: rectangles updated on all 6 pages, all 4 cuts per page
kept including both stepped ones, metadata intact, and exported slices
scale larger now that the margin shadow no longer pads the width.

--force remains the destructive option that re-detects everything.

noteman_slicer/cli.py

@@ -61,6 +61,14 @@ def _project(args: argparse.Namespace) -> int:
         print(f"{path.name}: loaded")
         if project.source_changed():
             print("  WARNING: the PDF has changed since these cuts were made")
+        if args.refit:
+            # Re-propose the content rectangle without disturbing cuts,
+            # discards or metadata — for projects made before detection
+            # proposed one, or whose rectangle was dragged wrong.
+            for i in range(len(project.pages)):
+                gray = page_raster(source, i)
+                project.pages[i].content_rect = detect_page(gray).content
+            print(f"  content rectangle re-fitted on {len(project.pages)} pages")
     else:
         detections, heights = [], []
         for i in range(len(source)):
@@ -145,6 +153,11 @@ def main(argv: list[str] | None = None) -> int:
     proj.add_argument("pdf")
     proj.add_argument("--save", action="store_true", help="write the project file")
     proj.add_argument("--force", action="store_true", help="re-detect, discarding existing state")
+    proj.add_argument(
+        "--refit",
+        action="store_true",
+        help="re-propose the content rectangle, keeping cuts and metadata",
+    )
     proj.add_argument("--type", choices=[t.value for t in SourceType])
     proj.set_defaults(func=_project)