Remove remaining OS-specific hardcodes from core scripts

OS version reads now go through platform_get_os_version() and
platform_os_version_probe_cmd() instead of grepping /etc/unraid-version directly.

STATE_DIR fallbacks to /boot/config removed — STATE_DIR is always set by
load_config.sh and the fallback encoded a platform-specific path.

Setup DB path references use platform_setup_db_path() instead of the
VARAVERK_SETUP_FILE/-/boot/config compound fallback.

DOCKER_APPDATA_BASE default removed from arr_sync.sh — the adapter sets it.

Partnership/partnership_onboard.sh

@@ -220,7 +220,7 @@ START=$(date +%s)
 write_onboard_phase() {
     local target_id="$1" phase="$2"
     local key="${target_id}_PHASE${phase}_DONE"
-    local state_file="${VARAVERK_SETUP_FILE:-${STATE_DIR:-/boot/config}/varaverk_setup.db}"
+    local state_file="$(platform_setup_db_path)"
     [[ "$DRY_RUN" == true ]] && { warn "DRY RUN — would write ${key}=true"; return 0; }
     if grep -q "^${key}=" "$state_file" 2>/dev/null; then
         sed -i "s|^${key}=.*|${key}=true|" "$state_file"
@@ -412,7 +412,7 @@ elif [[ "$PHASE1_ONLY" == true ]]; then
                 # Write key-ready flag so UI can show the manual-install state
                 [[ "$DRY_RUN" == false ]] && {
                     local kflag="${MIRROR_ID}_KEY_READY"
-                    local _setup_f="${VARAVERK_SETUP_FILE:-${STATE_DIR:-/boot/config}/varaverk_setup.db}"
+                    local _setup_f="$(platform_setup_db_path)"
                     grep -q "^${kflag}=" "$_setup_f" 2>/dev/null \
                         && sed -i "s|^${kflag}=.*|${kflag}=true|" "$_setup_f" \
                         || echo "${kflag}=true" >> "$_setup_f"