feat: add field validation API with signup form example

Introduces Field, Rule, ValidateAll, ResetFields, and AddError for
declarative input validation. Includes built-in rules (Required,
MinLen, MaxLen, Min, Max, Email, Pattern, Custom) and a signup
example exercising the full API surface.

rule.go

@@ -0,0 +1,128 @@
+package via
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// Rule defines a single validation check for a Field.
+type Rule struct {
+	validate func(val string) error
+}
+
+// Required rejects empty or whitespace-only values.
+func Required(msg ...string) Rule {
+	m := "This field is required"
+	if len(msg) > 0 {
+		m = msg[0]
+	}
+	return Rule{func(val string) error {
+		if strings.TrimSpace(val) == "" {
+			return fmt.Errorf("%s", m)
+		}
+		return nil
+	}}
+}
+
+// MinLen rejects values shorter than n characters.
+func MinLen(n int, msg ...string) Rule {
+	m := fmt.Sprintf("Must be at least %d characters", n)
+	if len(msg) > 0 {
+		m = msg[0]
+	}
+	return Rule{func(val string) error {
+		if len(val) < n {
+			return fmt.Errorf("%s", m)
+		}
+		return nil
+	}}
+}
+
+// MaxLen rejects values longer than n characters.
+func MaxLen(n int, msg ...string) Rule {
+	m := fmt.Sprintf("Must be at most %d characters", n)
+	if len(msg) > 0 {
+		m = msg[0]
+	}
+	return Rule{func(val string) error {
+		if len(val) > n {
+			return fmt.Errorf("%s", m)
+		}
+		return nil
+	}}
+}
+
+// Min parses the value as an integer and rejects values less than n.
+func Min(n int, msg ...string) Rule {
+	m := fmt.Sprintf("Must be at least %d", n)
+	if len(msg) > 0 {
+		m = msg[0]
+	}
+	return Rule{func(val string) error {
+		v, err := strconv.Atoi(val)
+		if err != nil {
+			return fmt.Errorf("%s", m)
+		}
+		if v < n {
+			return fmt.Errorf("%s", m)
+		}
+		return nil
+	}}
+}
+
+// Max parses the value as an integer and rejects values greater than n.
+func Max(n int, msg ...string) Rule {
+	m := fmt.Sprintf("Must be at most %d", n)
+	if len(msg) > 0 {
+		m = msg[0]
+	}
+	return Rule{func(val string) error {
+		v, err := strconv.Atoi(val)
+		if err != nil {
+			return fmt.Errorf("%s", m)
+		}
+		if v > n {
+			return fmt.Errorf("%s", m)
+		}
+		return nil
+	}}
+}
+
+// Pattern rejects values that don't match the regular expression re.
+func Pattern(re string, msg ...string) Rule {
+	m := "Invalid format"
+	if len(msg) > 0 {
+		m = msg[0]
+	}
+	compiled := regexp.MustCompile(re)
+	return Rule{func(val string) error {
+		if !compiled.MatchString(val) {
+			return fmt.Errorf("%s", m)
+		}
+		return nil
+	}}
+}
+
+var emailRegexp = regexp.MustCompile(`^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$`)
+
+// Email rejects values that don't look like an email address.
+func Email(msg ...string) Rule {
+	m := "Invalid email address"
+	if len(msg) > 0 {
+		m = msg[0]
+	}
+	return Rule{func(val string) error {
+		if !emailRegexp.MatchString(val) {
+			return fmt.Errorf("%s", m)
+		}
+		return nil
+	}}
+}
+
+// Custom creates a rule from a user-provided validation function.
+// The function should return nil for valid input and an error for invalid input.
+func Custom(fn func(string) error) Rule {
+	return Rule{validate: fn}
+}