[Feature] ERC/DRC reports shall list what severities are included

Reports with no entries can be very misleading, as reports are possibly filtered. This means a "valid" report can be generated showing no violations even though some exist. The CLI explicitly supports --severity-* options and also the ERC/DRC dialogs support filtering by severity before saving a report. However when inspecting a report afterwards one does not know what kind of severities are included in it (unless there is one of each type that was included).

Application: kicad-cli x86_64 on x86_64

Version: 9.99.0-4139-ge0760b90f7-dirty, release build

Libraries:
        wxWidgets 3.2.8
        FreeType 2.13.3
        HarfBuzz 10.2.0
        FontConfig 2.15.0
        libcurl/8.14.1 OpenSSL/3.5.4 zlib/1.3.1 brotli/1.1.0 zstd/1.5.7 libidn2/2.3.8 libpsl/0.21.2 libssh2/1.11.1 nghttp2/1.64.0 nghttp3/1.8.0 librtmp/2.3 OpenLDAP/2.6.10

Platform: Debian GNU/Linux 13 (trixie), 64 bit, Little endian, wxBase, KDE, x11

Build Info:
        Date: Nov 22 2025 22:11:16
        wxWidgets: 3.2.8 (wchar_t,wx containers) GTK+ 0.0
        Boost: 1.83.0
        OCC: 7.8.1
        Curl: 8.14.1
        ngspice: 45.2
        Compiler: GCC 14.2.0 with C++ ABI 1019
        KICAD_IPC_API=ON
        KICAD_USE_PCH=OFF

Locale: 
        Lang: en_US
        Enc: UTF-8
        Num: 1.234,5
        Encoded кΩ丈: D0BACEA9E4B888 (sys), D0BACEA9E4B888 (utf8)