refactor(config): rewrite config parser

This PR refactors the configuration parser, replacing the old parser implementation with a new, more structured approach that includes validation and improved organization.

Key changes:
- Complete rewrite of the configuration parser using a map-based structure with built-in validation
- Addition of comprehensive validator functions for configuration values
- Renamed numerous configuration getter methods for better consistency

internal/config/validators.go

@@ -0,0 +1,60 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package config // import "miniflux.app/v2/internal/config"
+
+import (
+	"fmt"
+	"slices"
+	"strconv"
+	"strings"
+)
+
+func validateChoices(rawValue string, choices []string) error {
+	if !slices.Contains(choices, rawValue) {
+		return fmt.Errorf("value must be one of: %v", strings.Join(choices, ", "))
+	}
+	return nil
+}
+
+func validateListChoices(inputValues, choices []string) error {
+	for _, value := range inputValues {
+		if !slices.Contains(choices, value) {
+			return fmt.Errorf("value must be one of: %v", strings.Join(choices, ", "))
+		}
+	}
+	return nil
+}
+
+func validateGreaterThan(rawValue string, min int) error {
+	intValue, err := strconv.Atoi(rawValue)
+	if err != nil {
+		return fmt.Errorf("value must be an integer")
+	}
+	if intValue > min {
+		return nil
+	}
+	return fmt.Errorf("value must be at least %d", min)
+}
+
+func validateGreaterOrEqualThan(rawValue string, min int) error {
+	intValue, err := strconv.Atoi(rawValue)
+	if err != nil {
+		return fmt.Errorf("value must be an integer")
+	}
+	if intValue >= min {
+		return nil
+	}
+	return fmt.Errorf("value must be greater or equal than %d", min)
+}
+
+func validateRange(rawValue string, min, max int) error {
+	intValue, err := strconv.Atoi(rawValue)
+	if err != nil {
+		return fmt.Errorf("value must be an integer")
+	}
+	if intValue < min || intValue > max {
+		return fmt.Errorf("value must be between %d and %d", min, max)
+	}
+	return nil
+}