v0.27.0: six-digit document codes for voice calls

• Every share link has a six-digit document code ("open document 437301" on a call). It lasts as long as the link: choose an expiry (never, 1, 7, 30 or 90 days) when you create it, and revoking the link ends the code.
• readm3.com/c/ and GET /api/v1/codes/ open the document read-only, no account needed.
• Codes are unique among working links (900,000 possible, no leading zero), freed when a link expires, and lookups are rate-limited.
• Also since v0.26.0: desktop files read from disk open and sync after a restart; shared images and PDFs carry their bytes.