v0.16.0: files up to 5 GB, no file limit

Signed in, the free tier opens any number of files of up to 5 GB each, with 5 GB of end-to-end encrypted synced storage; more is pay as you go ($0.05 per GB a month over 5 GB) from a CoinPay balance. Files over 4 MB open read-only and download exactly. The desktop app no longer stops at 1,000 files and reads large files in ranges. Signed out, the local limits stay at 4 MB per file, 20 MB and 1,000 files. (#43)