build: auto-wire QT_PLUGIN_PATH for a user-local install

A home-prefix install puts the plugins outside Qt's default search path, so
Kate would not list them until QT_PLUGIN_PATH includes the install dir — the
step a bare 'cmake --install' silently left undone. Solve it the way the ollie
project does (a Plasma login env snippet), and automate the common case:

- Add 99-deft.sh.in, a Plasma env snippet that prepends the install's qt6
  plugin dir to QT_PLUGIN_PATH. configure_file() fills in the absolute path.
- On a user-local install (prefix inside $HOME), install(CODE) drops the
  configured 99-deft.sh into ~/.config/plasma-workspace/env/ so the lookup
  path is wired up with no manual step; Plasma sources it at next login. The
  installer also prints a one-line export to use the plugins immediately.
- On a system-prefix install it does not touch the user's config; it just
  prints where the plugins went and leaves the ready snippet in the build dir.

README: add a 'Making Kate find a user-local install' section explaining that
QT_PLUGIN_PATH must include ~/.local, that the installer handles it, and how to
activate it in the current session (or a non-Plasma shell).

Verified: cmake --install with the default prefix installs the six plugins and
writes ~/.config/plasma-workspace/env/99-deft.sh with the correct path.

99-deft.sh.in

@@ -0,0 +1,10 @@
+# Deft — add the user-local KTextEditor plugin directory to Qt's search path.
+#
+# Kate/KTextEditor only loads plugins found on QT_PLUGIN_PATH. When Deft is
+# installed under a user prefix (the default, ~/.local) rather than a system
+# one, that directory is not searched unless it is added here. Plasma sources
+# every *.sh in ~/.config/plasma-workspace/env/ at login.
+#
+# The path below is filled in at install time with the directory the plugins
+# were installed into.
+export QT_PLUGIN_PATH="@DEFT_PLUGIN_PATH@${QT_PLUGIN_PATH:+:$QT_PLUGIN_PATH}"

CMakeLists.txt

@@ -54,6 +54,37 @@ find_package(KF6 ${KF_MIN_VERSION} COMPONENTS TextEditor XmlGui CoreAddons I18n)
 set(KDE_INSTALL_PLUGINDIR "${QT6_INSTALL_PLUGINS}")
 set(KDE_INSTALL_PLUGINDIR "${QT6_INSTALL_PLUGINS}" CACHE PATH "" FORCE)
 
+# Qt only searches QT_PLUGIN_PATH for plugins. A user-prefix install (the
+# default) puts the plugins outside the system search path, so Kate will not
+# find them until that directory is added to QT_PLUGIN_PATH. Generate a Plasma
+# login environment snippet that does exactly that, and — for a user-local
+# install — drop it into ~/.config/plasma-workspace/env/ automatically so the
+# lookup path is wired up without a manual step. For a system install the user
+# installs it themselves (printed below); packagers usually don't want it.
+get_filename_component(DEFT_PLUGIN_PATH
+    "${CMAKE_INSTALL_PREFIX}/${QT6_INSTALL_PLUGINS}" ABSOLUTE)
+configure_file("${CMAKE_SOURCE_DIR}/99-deft.sh.in"
+    "${CMAKE_BINARY_DIR}/99-deft.sh" @ONLY)
+
+# Is the chosen prefix inside the user's home (a personal install)?
+string(FIND "${CMAKE_INSTALL_PREFIX}" "$ENV{HOME}" _deft_prefix_in_home)
+if(_deft_prefix_in_home EQUAL 0)
+    set(_deft_plasma_env_dir "$ENV{HOME}/.config/plasma-workspace/env")
+    install(CODE "
+        file(MAKE_DIRECTORY \"${_deft_plasma_env_dir}\")
+        file(INSTALL \"${CMAKE_BINARY_DIR}/99-deft.sh\"
+             DESTINATION \"${_deft_plasma_env_dir}\")
+        message(STATUS \"Deft: installed Qt plugin-path snippet to ${_deft_plasma_env_dir}/99-deft.sh\")
+        message(STATUS \"Deft: log out and back in (or run: export QT_PLUGIN_PATH=\\\"${DEFT_PLUGIN_PATH}:\\\$QT_PLUGIN_PATH\\\") so Kate finds the plugins now.\")
+    ")
+else()
+    install(CODE "
+        message(STATUS \"Deft: plugins installed under a system prefix; QT_PLUGIN_PATH already covers it if that prefix is standard.\")
+        message(STATUS \"Deft: if Kate does not list them, add this to the Qt plugin path: ${DEFT_PLUGIN_PATH}\")
+        message(STATUS \"Deft: a ready-made login snippet is at ${CMAKE_BINARY_DIR}/99-deft.sh (copy to ~/.config/plasma-workspace/env/).\")
+    ")
+endif()
+
 enable_testing()
 
 add_subdirectory(src)

README.md

@@ -89,6 +89,28 @@ system-wide instead, pass an explicit prefix (and expect to need `root`):
 cmake --install build --prefix /usr
 ```
 
+### Making Kate find a user-local install
+
+Qt only loads plugins from directories on **`QT_PLUGIN_PATH`**, and that does
+*not* include `~/.local` by default — so after a home-directory install Kate
+will not list the plugins until that path is on the search list. This is a
+one-time, per-user setup step.
+
+The install **automates it for you**: when the prefix is inside your home, it
+writes a login snippet to
+`~/.config/plasma-workspace/env/99-deft.sh` that adds the plugin directory to
+`QT_PLUGIN_PATH`. Plasma sources it at login, so after the next **log out / log
+in** Kate finds the plugins. To use them in the *current* session without
+logging out, run the export the installer prints, e.g.:
+
+```sh
+export QT_PLUGIN_PATH="$HOME/.local/lib/x86_64-linux-gnu/qt6/plugins:$QT_PLUGIN_PATH"
+```
+
+(For a non-Plasma session, put that same line in your shell profile. For a
+system-prefix install this step is unnecessary; the installer prints the ready
+snippet under the build dir if you still want it.)
+
 Then in Kate: Settings → Configure Kate → Plugins → enable each **`[deft:…]`**
 plugin you installed. See [The plugins](#the-plugins) below for what each one
 does and how to drive it.