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

README.md

@@ -103,7 +103,7 @@ Surveyed on this machine 2026-09-09 — **most of it is already installed**:
 
 ```bash
 sudo apt update && sudo apt install -y \
-  libqt6svg6-dev \    # GUI: SVG icon rendering
+  qt6-svg-dev \       # GUI: SVG icon rendering
   libsecret-1-dev \   # DAEMON: Secret Service for site logins
   nodejs npm \        # EXT + PROTO: extension build, mockd, conformance runner
   clang               # optional: libFuzzer targets in M7

docs/agents/AGENT-PKG-QA.md

@@ -14,7 +14,7 @@ Do not write feature code in any lane's directory.
 1. **Toolchain script** — `tools/bootstrap.sh` installing the `apt` list in the README,
    verified on a clean 26.04 VM. The dev machine already has git 2.53, CMake 4.2.3,
    g++ 15.2, ninja, Qt 6 dev, libcurl, SQLite, nlohmann-json and ffmpeg; only
-   `libqt6svg6-dev`, `libsecret-1-dev`, `nodejs`/`npm` and (optionally) `clang` are
+   `qt6-svg-dev`, `libsecret-1-dev`, `nodejs`/`npm` and (optionally) `clang` are
    missing. The script must still install the full list for clean machines.
    ⚠ CMake 4.x rejects `cmake_minimum_required` below 3.5 — check every dependency.
 2. **CMake** — top-level `CMakeLists.txt` + `CMakePresets.json` with presets:

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=(