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.

debianito.sh

@@ -116,6 +116,14 @@ kernels. Use the stable kernel provided by Bullseye." 10 60
 
 check_root
 check_sudo
+if ! _check_network; then
+    echo -e "${YELLOW}──────────────────────────────────────────${NC}"
+    echo -e "${YELLOW} No internet connectivity detected.${NC}"
+    echo -e "${YELLOW} Package installation will fail without network.${NC}"
+    echo -e "${YELLOW} You can use: System Info, User Privileges, and${NC}"
+    echo -e "${YELLOW} other offline features.${NC}"
+    echo -e "${YELLOW}──────────────────────────────────────────${NC}"
+fi
 check_system_time
 sync_system_time