Fit the page on startup, name the bundle, show the selection

Three things a first session trips over.

The editor opened at an arbitrary zoom. show_page does fit the page, but
it runs before the window has been laid out, when the viewport is still
its default size; redo it once when the real size arrives.

The bundle was named after the PDF, which is whatever the download was
called. Take the song's title instead — unsafe characters dropped, then
whitespace collapsed to dashes. Accented letters stay, since ä and ö are
not a filesystem's problem, but a leading dot would hide the file.

And the selected slice was drawn as an outline whose top and bottom edges
run under the cut lines painted over them, leaving two thin verticals in
the margins and no way to tell what was selected. Wash the slice, as the
discard and engraved states already do, and keep the outline for the trim
anomalies it exists to show.

noteman_slicer/bundle.py

@@ -13,6 +13,7 @@ a jump source's `destination`.
 from __future__ import annotations
 
 import json
+import re
 import zipfile
 from pathlib import Path
 
@@ -39,6 +40,19 @@ METADATA_FIELDS = (
 NUMERIC_FIELDS = frozenset({"tempo"})
 
 
+def filename(project: Project) -> str:
+    """The bundle's name, from the song's title.
+
+    Spaces become dashes and anything that is not a letter, digit, dash, dot or
+    underscore goes. Letters keep their accents — ä and ö are not a filesystem's
+    problem — but a leading dot would make the bundle invisible.
+    """
+    # Drop the unsafe characters before collapsing whitespace, not after, or
+    # "Sävel & Ääni" keeps the dash the ampersand left behind.
+    title = re.sub(r"[^\w\s.-]", "", (project.metadata.get("title") or ""))
+    return f"{re.sub(r'\s+', '-', title.strip()).lstrip('.-') or 'song'}.zip"
+
+
 def _engraving(project: Project, page: int, slot: int) -> dict | None:
     """The notation behind a re-engraved slice, or None for a scanned one.