Adds the fr_FR locale, a rebuilt admin settings page on the core admin
theme's own classes, Batch Manager clear/append/replace controls, and
chunked metadata writing via pwg.images.dcWriteMetadataBatch.

Security: every photo read and write now applies Piwigo's
get_sql_condition_FandF() visibility filter and fails closed, so the
admin and web-service surfaces can no longer act on a photo the caller
cannot see. Labels and help text are escaped before output, and
pwg.dc.setValues validates value type and length.

Fixes a data-loss bug where dc_clean_value() stripped backslashes from
values on their way into the file, so Windows paths and regexes were
written to XMP/IPTC with their backslashes removed.