Fix NVIDIA Kepler Logic & Kernel Headers

- Normalized PCI ID case sensitivity in `_helpers.sh`. The `detect_nvidia_arch()` function now converts input to uppercase (`${1^^}`) to match legacy architecture map keys, resolving "unknown" detection for all NVIDIA GPUs.
- Removed silent fallback to standard drivers in `gpu.sh` Bookworm logic. Kepler and other architectures now invoke specific installation functions directly without defaulting to unsupported driver versions (e.g., 535).
- Corrected NVIDIA legacy package name in `nvidia.sh`. Updated `_install_nvidia_bookworm_kepler()` to use `nvidia-tesla-470-driver` instead of deprecated Bullseye naming conventions for Debian 12.
- Replaced specific kernel header references (`linux-headers-$(uname -r)`) with meta-package `linux-headers-amd64` in `firmware.sh`. Fixes DKMS installation failures for Broadcom/WiFi drivers across multiple Debian versions.

modules/gpu.sh

@@ -181,11 +181,7 @@ _install_nvidia_stack() {
 
     elif [ "$DEBIAN_VERSION" = "12" ]; then
         if [ "$(is_nvidia_kepler)" = "true" ]; then
-            if type _install_nvidia_bookworm_kepler &>/dev/null; then
-                _install_nvidia_bookworm_kepler
-            else
-                _install_nvidia_standard
-            fi
+            _install_nvidia_bookworm_kepler
         elif [ "$(is_nvidia_fermi)" = "true" ]; then
             _msg "NVIDIA Fermi — Bookworm" \
                 "Fermi GPUs (GF1xx) are not supported\nin Debian 12 (Bookworm).\nThe nvidia-legacy-390xx driver is\nnot available in this version.\n\nNo NVIDIA driver will be installed."