pkg: fix qt6 svg dev package name — libqt6svg6-dev has no 26.04 candidate

`apt-cache policy libqt6svg6-dev` is "Candidate: (none)" on 26.04; the package
that carries the Svg headers and Qt6SvgConfig.cmake is qt6-svg-dev. Since
gui/CMakeLists.txt landed on main the root build's
find_package(Qt6 ... Svg REQUIRED) is live, so a clean 26.04 box could not
configure the project at all. Same name in README.md and AGENT-PKG-QA.md.

Reported in gui/docs/pkg-qa-requests-m1.md R1.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_0143aKiohmDiyefJBwHDJJqw

tools/bootstrap.sh

@@ -84,7 +84,9 @@ APT_GUI=(
     qt6-base-dev
     qt6-tools-dev
     qt6-tools-dev-tools        # lrelease/lupdate for i18n
-    libqt6svg6-dev             # GUI: SVG icon rendering
+    qt6-svg-dev                # GUI: SVG icon rendering. Was libqt6svg6-dev, which has no
+                              #      candidate on 26.04; qt6-svg-dev is the package that
+                              #      ships the headers + Qt6SvgConfig.cmake find_package needs.
 )
 
 APT_LINT=(