Build and install cosmic-conf and the shared assets from the top-level justfile

`just build` built 26 upstream components and our two forks. `just install`
installed the same set. Neither touched cosmic-conf or anything under config/,
which meant the recipe that is supposed to produce a desktop produced one with
no config compiler, no bar layout, no rofi theme and no power menu -- a
HyprCosmic session that comes up to a blank screen and cannot be logged out of.

So three lines, in the places the existing recipes already establish:

  build     cargo build --release --manifest-path cosmic-conf/Cargo.toml
  install   the binary to $prefix/bin, then tools/install-assets.sh
  clean     cosmic-conf/target

cargo directly rather than `just cosmic-conf/build-release`, because cosmic-conf
is a crate in this repository rather than a submodule and has no Justfile of its
own to delegate to.

install-assets.sh runs last and with --no-session. Last because it is the only
step whose output is worth reading: several of the files it places name
/usr/share/hyprcosmic as a literal -- a .rasi has no variables and the autostart
file is deliberately not a shell -- so a prefix other than /usr installs them
where nothing will look, and the script says which ones. --no-session because
cosmic-session/install has already placed start-hyprcosmic and
hyprcosmic.desktop by then, and installing them twice would leave it unclear
which recipe owns them.

Verified by running the two new install lines verbatim into a staging root:
eight files, the cosmic-conf binary and the seven shared assets, and no session
entry point among them.

justfile

@@ -8,6 +8,10 @@ build:
     {{ just }} cosmic-applibrary/build-release
     {{ just }} cosmic-bg/build-release
     {{ make }} -C cosmic-comp all
+    # cargo directly, not `just cosmic-conf/build-release`: cosmic-conf is not a
+    # submodule, it is a crate in this repository, and it has no Justfile of its
+    # own to delegate to.
+    cargo build --release --manifest-path cosmic-conf/Cargo.toml
     {{ just }} cosmic-edit/build-release
     {{ just }} cosmic-files/build-release
     {{ just }} cosmic-greeter/build-release
@@ -36,6 +40,7 @@ install rootdir="" prefix="/usr/local": build
     {{ just }} rootdir={{rootdir}} prefix={{prefix}} cosmic-applibrary/install
     {{ just }} rootdir={{rootdir}} prefix={{prefix}} cosmic-bg/install
     {{ make }} -C cosmic-comp install DESTDIR={{rootdir}} prefix={{prefix}}
+    install -Dm0755 cosmic-conf/target/release/cosmic-conf {{rootdir}}{{prefix}}/bin/cosmic-conf
     {{ just }} rootdir={{rootdir}} prefix={{prefix}} cosmic-edit/install
     {{ just }} rootdir={{rootdir}} prefix={{prefix}} cosmic-files/install
     {{ just }} rootdir={{rootdir}} prefix={{prefix}} cosmic-greeter/install
@@ -59,6 +64,16 @@ install rootdir="" prefix="/usr/local": build
     {{ make }} -C cosmic-workspaces-epoch install DESTDIR={{rootdir}} prefix={{prefix}}
     {{ just }} rootdir={{rootdir}} pop-launcher/install
     {{ make }} -C xdg-desktop-portal-cosmic install DESTDIR={{rootdir}} prefix={{prefix}}
+    # The waybar and rofi assets, and the power menu. Last, because it is the
+    # only step that prints a warning worth reading: several of these files name
+    # /usr/share/hyprcosmic as a literal -- a .rasi has no variables and the
+    # autostart file is not a shell -- so a prefix other than /usr installs them
+    # somewhere they will not be looked for. The script says which files.
+    #
+    # --no-session because cosmic-session/install above already placed
+    # start-hyprcosmic and hyprcosmic.desktop, and installing them twice would
+    # only make it unclear which recipe owns them.
+    PREFIX={{prefix}} DESTDIR={{rootdir}} ./tools/install-assets.sh --no-session
 
 _mkdir dir:
    mkdir -p dir
@@ -80,6 +95,7 @@ clean:
     rm -rf cosmic-applibrary/target
     rm -rf cosmic-bg/target
     rm -rf cosmic-comp/target
+    rm -rf cosmic-conf/target
     rm -rf cosmic-edit/target
     {{ just }} cosmic-files/clean
     rm -rf cosmic-greeter/target