feat: auto-track fields on context for zero-arg ValidateAll/ResetFields

Fields created via Context.Field are now tracked on the page context,
so ValidateAll() and ResetFields() with no arguments operate on all
fields by default. Explicit field args still work for selective use.

Also switches MinLen/MaxLen to utf8.RuneCountInString for correct
unicode handling and replaces fmt.Errorf with errors.New where
format strings are unnecessary.

context.go

@@ -32,6 +32,7 @@ type Context struct {
 	mu                sync.RWMutex
 	ctxDisposedChan   chan struct{}
 	reqCtx            context.Context
+	fields            []*Field
 	subscriptions     []Subscription
 	subsMu            sync.Mutex
 	disposeOnce       sync.Once
@@ -482,16 +483,28 @@ func (c *Context) unsubscribeAll() {
 
 // Field creates a signal with validation rules attached.
 // The initial value seeds both the signal and the reset target.
+// The field is tracked on the context so ValidateAll/ResetFields
+// can operate on all fields by default.
 func (c *Context) Field(initial any, rules ...Rule) *Field {
-	return &Field{
+	f := &Field{
 		signal:     c.Signal(initial),
 		rules:      rules,
 		initialVal: initial,
 	}
+	target := c
+	if c.isComponent() {
+		target = c.parentPageCtx
+	}
+	target.fields = append(target.fields, f)
+	return f
 }
 
-// ValidateAll runs Validate on every field, returning true only if all pass.
+// ValidateAll runs Validate on each field, returning true only if all pass.
+// With no arguments it validates every field tracked on this context.
 func (c *Context) ValidateAll(fields ...*Field) bool {
+	if len(fields) == 0 {
+		fields = c.fields
+	}
 	ok := true
 	for _, f := range fields {
 		if !f.Validate() {
@@ -501,8 +514,12 @@ func (c *Context) ValidateAll(fields ...*Field) bool {
 	return ok
 }
 
-// ResetFields resets every field to its initial value and clears errors.
+// ResetFields resets each field to its initial value and clears errors.
+// With no arguments it resets every field tracked on this context.
 func (c *Context) ResetFields(fields ...*Field) {
+	if len(fields) == 0 {
+		fields = c.fields
+	}
 	for _, f := range fields {
 		f.Reset()
 	}