Drag-dropping page images then clicking a nav link silently discarded the
whole form. Add a reusable guardUnsavedChanges helper (SvelteKit beforeNavigate
confirm + native beforeunload) and wire it into the manga upload, manga edit,
and chapter upload forms. The dirty check returns false while submitting so the
post-save redirect isn't prompted.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>