[harmony] project filenames are invalid on Windows because of date formatting

[suddjian]

I'm using Syncthing to sync between my reMarkable and Windows. Harmony adds the current date/time to the filename, which includes colons. Windows doesn't allow colons in filenames, so the sync fails.

I recommend formatting the date using underscores for broader compatibility.

Harmony inserts the date here:

https://github.com/rmkit-dev/rmkit/blob/1de13cf4ddfb11e65b67b567872db253e6b1f54f/src/harmony/app/canvas.cpy#L389-L392

The error logged by Syncthing, for additional context:

2024-03-24 18:00:31 Puller (folder "reMarkable Harmony Drawings" (redacted), item "Sketches.2024-03-23-22:50:39.hrm"): syncing: name is invalid, contains Windows reserved character: ":"