test(config): add comprehensive tests for config loading path

Add tests covering the main configuration loading path which was
previously untested (only ClassifyTargets and PSL logic had tests).

New test coverage:
- New() constructor with default values, env overrides, and error cases
- buildConfig() interval parsing with fallback to defaults on invalid input
- setupViper() env prefix binding and default value registration
- parseCSV() edge cases: whitespace, trailing/leading commas, empty segments
- configureDebugLogging() debug mode activation
- StatePath() path construction with various DataDir values
- ErrNoTargets sentinel error for missing/empty targets
- PORT env without DNSWATCHER_ prefix (compatibility)
- Target classification into domains vs hostnames via PSL
- Invalid targets (public suffixes) rejected with clear error

Coverage for internal/config increased from 23% to 92.5%.

internal/config/export_test.go

@@ -0,0 +1,6 @@
+package config
+
+// ParseCSVForTest exports parseCSV for use in external tests.
+func ParseCSVForTest(input string) []string {
+	return parseCSV(input)
+}