fix: Add default value for AVAILABLE_SPACE to prevent TypeError

Fix crash when df produces unexpected output that results in empty
AVAILABLE_SPACE variable, causing 'integer expression expected' error.

Problem:
- df may produce unexpected output format (different locale, unusual
  filesystem name spanning lines, or non-standard df implementation)
- While '|| echo "0"' handles pipeline failures, it doesn't trigger if
  awk succeeds but produces no output (empty string)
- When AVAILABLE_SPACE is empty, comparison [ "$AVAILABLE_SPACE" -lt 500 ]
  fails with 'integer expression expected' error
- With set -e, this causes script to exit unexpectedly

Solution:
- Add AVAILABLE_SPACE=${AVAILABLE_SPACE:-0} before comparison to ensure
  variable always has a numeric value (defaults to 0 if empty)
- This gracefully handles edge cases where df/awk produces unexpected output

scripts/install/one-shot-install.sh

@@ -122,6 +122,8 @@ check_disk_space() {
     
     # Check available space in MB
     AVAILABLE_SPACE=$(df -m / | awk 'NR==2{print $4}' || echo "0")
+    # Ensure AVAILABLE_SPACE has a default value if empty (handles unexpected df output)
+    AVAILABLE_SPACE=${AVAILABLE_SPACE:-0}
     
     if [ "$AVAILABLE_SPACE" -lt 500 ]; then
         print_error "Insufficient disk space: ${AVAILABLE_SPACE}MB available (need at least 500MB)"