fix backports and system modules

- Refactored `is_backports_enabled()` to dynamically detect backports using `DEBIAN_CODENAME` instead of hardcoded strings, improving script portability.
- Added support for standalone classic backports files (`/etc/apt/sources.list.d/debian-backports.list`) and deb822 format (`*.sources`) to ensure consistent detection across different repository configurations.
- Resolved Mesa and Kernel backports detection bugs, ensuring correct dialogue prompts and preventing false error messages during installation.
- Implemented smart rounding for zram memory allocation to the nearest Gigabyte for optimized performance.
- Enhanced Whiptail manual input validation for zram to block empty, non-numeric, or zero values.
- Introduced a dedicated `software_centers.sh` module for the modular and clean installation of GNOME Software and Plasma Discover.
- Added smart desktop environment detection (GTK/QT) to automatically suggest and install the optimal software center.
- Implemented a two-phase installation flow for software centers (Base Store -> Optional Flatpak support) with static consistency warnings based on the detected desktop environment.
- update README.md

modules/extras/system/software_centers.sh

@@ -0,0 +1,40 @@
+# software_centers.sh — Standalone Software Center installer
+
+_cat_software_centers() {
+    local de_type
+    de_type=$(_detect_desktop_type)
+    local sc_choice
+    sc_choice=$(whiptail --title "Software Centers" --menu \
+        "Choose a software store to install:" 12 65 2 \
+        "gnome-software"   "Software Center for GNOME" \
+        "plasma-discover"  "Software manager for Plasma" \
+        3>&1 1>&2 2>&3)
+    [ -z "$sc_choice" ] && return
+
+    if { [ "$de_type" = "qt" ] && [ "$sc_choice" = "gnome-software" ]; } || \
+       { [ "$de_type" = "gtk" ] && [ "$sc_choice" = "plasma-discover" ]; }; then
+        _msg "Warning" "Warning: This store requires extra background libraries \
+and may look visually inconsistent with your current desktop environment."
+        ! _confirm "Continue?" "Install anyway?" && return
+    fi
+
+    _run_cmd "Install" "sudo apt install -y $sc_choice" "Installing ${sc_choice}..."
+
+    if _confirm "Flatpak Support" "Do you want to enable Flatpak support for this software center?"; then
+        local bpkg
+        if [ "$sc_choice" = "gnome-software" ]; then
+            bpkg="gnome-software-plugin-flatpak"
+        else
+            bpkg="plasma-discover-backend-flatpak"
+        fi
+        if ! is_installed "flatpak"; then
+            _run_cmd "Flatpak" "sudo apt install -y flatpak" "Installing Flatpak..."
+        fi
+        _run_cmd "Plugin" "sudo apt install -y $bpkg" "Installing $bpkg..."
+        flatpak remote-add --if-not-exists flathub \
+            https://dl.flathub.org/repo/flathub.flatpakrepo
+        echo "Flathub repository added."
+    fi
+
+    echo -e "${GREEN}Software store installed.${NC}"
+}