feat: extract .env validators to common.sh and add validate_env()

Move 10 validation functions from configure_env.sh to lib/common.sh as
shared utilities. Define variable-to-validator mapping using parallel
arrays (bash 3.2 compatible). validate_env() checks all ~50 .env
variables against their expected format and reports all failures at once.

Wired into preflight.sh (Check 6b) and bitwarden_to_env.sh (post-restore).
configure_env.sh now sources validators from common.sh instead of
defining its own copies.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

setup/bitwarden_to_env.sh

@@ -156,4 +156,15 @@ else
 fi
 
 log_success "Restored .env to $OUTPUT_FILE ($field_count variables)"
-log_warn "Review the file before use — check for placeholder values that may need updating"
+
+# Validate restored values against expected formats
+log_info "Validating restored .env values..."
+set -a
+# shellcheck source=/dev/null
+source "$OUTPUT_FILE"
+set +a
+if validate_env; then
+  log_success "All .env values pass format validation"
+else
+  log_warn "Some .env values failed validation — review and fix before running preflight"
+fi