- Downstream version and the upstream revision it's based on
  (commit and date)

- URL to our modified source code.

- miniaudio and libsodium versions.

- Build profile (release, dev, or debug).

Also, stop checking for updates.

We only support Qt 6, remove all conditional code that was dependent on
earlier versions.

Let users explicitly enable logging if they so desire. Don't clutter the
disk space and perform I/O for each line when the rest are unaware.

Demo server was being deleted and recreated every time the lobby was
constructed, so it was always active. This rendered the "are we playing
a demo" checks useless as they always assumed we did. Logs didn't work
because of it, for example.

Construct the demo server only when the user selects demo playback, and
destroy it during the destruction of courtroom.

Make log_filename based on the application path, so `logs` subdirectory
is created relative to the executable rather than the working directory.

Remove the error dialog that pops up if an image format is missing.

Check AVIF, WebP, and APNG support, and report their status. As it
stands, WebP should be supported by Qt itself, and AVIF has yet to be
added. The rest (like PNG and JPEG) are on by default.

configure.sh isn't used to build this.

README_LINUX.md is inapplicable.

The subprotocol shall use binary frames, and AO protocol stays separated
within the text frames.

Introduce start_auth_flow, a function invoked by typing `/auth username`
in OOC. It sends an public-key authentication request to the server,
starting the entire flow.

The flow invoves two dialogs: to select the key, and to enter the
passphrase to unlock the key. For convenience, each successful unlock
also remembers the key for that username on the server, storing this
in `saved_auth.json` (I chose JSON because I wanted it to stay
human-editable; INI would be better, but it suffers from bad platform
quirks in Qt).