The <a download> and <a target="_blank"> patterns don't work in
WKWebView. Add downloadTextFile() and openUrl() utilities in
src/lib/downloadFile.ts that use @capacitor/filesystem and
@capacitor/share on native platforms, falling back to standard
browser behavior on web.
Update all call sites: onboarding key download (InitialSyncGate,
SignupDialog), image lightbox buttons (ImageGallery, ProfilePage).
Document Capacitor compatibility constraints in AGENTS.md.
Alex Gleason
da27054a9b