lang & firefox-esr

- Added `_detect_lang()` helper to `modules/utils.sh` (extracts first 2 chars from `$LANG`, fallback to `en`).
- Created `_detect_lang_pkg()` utility for LibreOffice localization: handles regional variants (`es-ar` → `es`) and skips English packages.
- Implemented `_check_network()` with cascade methods (ping→wget→curl) in `modules/utils.sh`. Warning-only pre-flight check after `check_sudo` in `debianito.sh`, compatible with `set -euo pipefail`.
- Fixed system time validation: changed `2026` threshold to `2025` in `check_system_time()` (replaced self-referential `$(date +%Y)` which always evaluated false).
- Refactored LibreOffice language selection in `office.sh`: replaced 10 lines of inline logic with `_detect_lang_pkg "libreoffice-l10n"`.
- Overhauled Firefox ESR integration in `internet.sh`: added state detection, mutual exclusion guard (both Firefox variants selected → warning), new `install_firefox_esr()` function with language support.
- Updated `install_firefox_mozilla()`: added Debian version guard (`<12`), replaced `_run_install` with `_run_cmd`.
- Standardized header in internet menu and improved whiptail UX for Firefox options.

modules/extras/office/office.sh

@@ -33,16 +33,8 @@ install_libreoffice_bpo() {
         return
     fi
 
-    local sys_lang
-    sys_lang=$(echo "${LANG:-en}" | cut -c1-2 | tr '[:upper:]' '[:lower:]')
-
-    local lang_pkg=""
-    if [ "$sys_lang" != "en" ]; then
-        local candidate="libreoffice-l10n-${sys_lang}"
-        if apt-cache show "$candidate" &>/dev/null 2>&1; then
-            lang_pkg="$candidate"
-        fi
-    fi
+    local lang_pkg
+    lang_pkg=$(_detect_lang_pkg "libreoffice-l10n")
 
     local ver
     ver=$(apt-cache policy libreoffice 2>/dev/null | awk 'NR==3 {print $2; exit}')