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.

field.go

@@ -0,0 +1,58 @@
+package via
+
+// Field is a signal with built-in validation rules and error state.
+// It embeds *signal, so all signal methods (Bind, String, Int, Bool, SetValue, Text, ID)
+// work transparently.
+type Field struct {
+	*signal
+	rules      []Rule
+	errors     []string
+	initialVal any
+}
+
+// Validate runs all rules against the current value.
+// Clears previous errors, populates new ones, returns true if all rules pass.
+func (f *Field) Validate() bool {
+	f.errors = nil
+	val := f.String()
+	for _, r := range f.rules {
+		if err := r.validate(val); err != nil {
+			f.errors = append(f.errors, err.Error())
+		}
+	}
+	return len(f.errors) == 0
+}
+
+// HasError returns true if this field has any validation errors.
+func (f *Field) HasError() bool {
+	return len(f.errors) > 0
+}
+
+// FirstError returns the first validation error message, or "" if valid.
+func (f *Field) FirstError() string {
+	if len(f.errors) > 0 {
+		return f.errors[0]
+	}
+	return ""
+}
+
+// Errors returns all current validation error messages.
+func (f *Field) Errors() []string {
+	return f.errors
+}
+
+// AddError manually adds an error message (useful for server-side or cross-field validation).
+func (f *Field) AddError(msg string) {
+	f.errors = append(f.errors, msg)
+}
+
+// ClearErrors removes all validation errors from this field.
+func (f *Field) ClearErrors() {
+	f.errors = nil
+}
+
+// Reset restores the field value to its initial value and clears all errors.
+func (f *Field) Reset() {
+	f.SetValue(f.initialVal)
+	f.errors = nil
+}