the donation button was not distributed anywhere

the donation button was not distributed anywhere

index.php reads it from FTOK_PATH/brisk_donate.txt, but that file was not in
the repository: whoever installed from scratch had no button, and the daemon
wrote a file_get_contents warning on every page load.

The html fragment now lives in data/, next to the other data files of the
installation, and INSTALL.sh copies it into FTOK_PATH only if it is not there
already, so as not to overwrite the one of the installation.

The file stays optional: index.php checks that it exists before reading it,
so whoever does not want the button does not get a dirty log (the code
already handled the case, but only after the warning had been written).

Checked in the container with a full installation, no longer just -W: the
file lands in FTOK_PATH, the page shows the form, the log stays clean;
without the file the page still loads with no form and no warning; and a
second installation does not touch the file already there.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014M1jiEq9cHdE5SE5j6vFuE