Help prevent plan-loss, Phase 2: Save last draft to localStorage

[mrwweb]

Building on #20, the next step is to save a copy of the plan to localStorage during editing so that it can be restored if lost.

Rough first thought of how to do this:

1. Save local copy of plan text to localStorage every ~15 seconds
2. If the saved copy doesn't match the plan editor when the page first loads, show a notice with a button to: restore the old copy

Question: Do I ever discard the saved copy? Maybe there's a "dismiss" x button that can delete the message by hiding the notice. Not totally sure.

[mrwweb]

How will I determine when a plan autosave is stale?

• I can use (pseudo-code) new Date({<span class="short">}.textContent).valueOf() to get the UTC and compare that to the current date/time in America/Chicago. If it's more than a day, discard?
• When the plan updated message loads, discard any autosave text?