v2.20: Safer when things go wrong

A third test run, this time as a hostile user, with things failing, with a year's worth of data, and through a first-time visitor's eyes.

• A save that fails (offline, signed out, or during an update) keeps what you typed and says what happened, instead of showing an error page.
• Previewing a Jira search no longer changes which Jira site the project sends to; only importing does, and a project stays with one site.
• Exported CSVs can't hide a spreadsheet formula behind a list bullet.
• A long link or word wraps instead of pushing the page sideways on a phone.
• Invisible characters copied along with a key are dropped, and an odd key like STORY-9999999999999999 can't block new ones.
• A sprint's day count uses your own date, so it's right in New Zealand's morning too.
• The limits on projects, stories and sprints hold even for requests sent at the same moment, with a cap on stored snapshots and on heavy actions in a few minutes.
• A Reconnect Jira button appears when the connection has expired; Send to Jira stops when Jira is busy or slow and says what's left to send.
• Emoji are never cut in half, deep Jira bullets can't push text off screen, and a link that names one site but opens another shows where it really goes.
• Each public page has its own title and description for search results and link previews, with a sitemap and a home-screen icon.
• The FAQ says plainly that the rules read English.