fix: robustness fixes from the project review

- autosave: a manifest that failed to save is retried with backoff instead
  of waiting for the next edit
- import: NaN and infinite numbers from broken files no longer reach the
  page file (they were stored as null and made the page unreadable)
- import: SVG images cannot load local files, Rnote page count is capped
- pdf: a panic in the PDF library fails one page or import instead of
  ending the render thread or the app
- storage: remove the temporary file when writing it fails

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

src/i18n/en.rs

@@ -133,6 +133,10 @@ pub const EN: &[(&str, &str)] = &[
     ("Fehler beim Schreiben auf die Festplatte: {e}", "Error writing to disk: {e}"),
     ("Fehler beim Umbenennen: {err}", "Error while renaming: {err}"),
     ("Fehler beim Wiederherstellen: {err}", "Error while restoring: {err}"),
+    (
+        "Das PDF kann nicht verarbeitet werden (interner Fehler)",
+        "The PDF cannot be processed (internal error)",
+    ),
     ("Fehler beim Öffnen: {err}", "Error while opening: {err}"),
     ("Fehler: {err}", "Error: {err}"),
     ("Fehlermeldung: {message}", "Error message: {message}"),