fix(compiler): never emit an empty pattern attribute

An empty pattern compiles to a regex matching only the empty string, so
every typed value becomes invalid and the form silently refuses to submit
-- no error, no request. @wrnexus/ui's input declares pattern: string = ""
and renders pattern="{pattern}", so every input that did not opt into a
pattern shipped one that could never match. This broke sign-up in a real
app, and only became visible once the dev-server client-module fix let
form enhancements mount at all.

Attributes reach the output through two emitters and both needed it: a
component's interpolated value is baked at render time, so the whole
attribute is now emitted by __wrnOptionalAttr, while a page's static
element is dropped at compile time. Component mounts are excluded, where
the value is a prop being passed down rather than an attribute.

minlength/maxlength/min/max/step/inputmode/accept get the same treatment --
inert when empty, but meaningless too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/compiler/src/codegen.ts

@@ -138,6 +138,29 @@ function isHtmlBooleanAttribute(name: string): boolean {
   return HTML_BOOLEAN_ATTRIBUTES.has(name.toLowerCase());
 }
 
+/*
+ * Constraint attributes that must be absent rather than empty. `pattern` is the
+ * dangerous one: an empty pattern compiles to a regex matching only the empty
+ * string, so every typed value becomes invalid and the form silently refuses to
+ * submit. A component declaring `pattern: string = ""` and rendering
+ * `pattern="{pattern}"` therefore breaks every input that did not ask for a
+ * pattern. The rest are inert when empty, but carry no meaning either.
+ */
+const OMIT_WHEN_EMPTY_ATTRIBUTES = new Set([
+  "pattern",
+  "minlength",
+  "maxlength",
+  "min",
+  "max",
+  "step",
+  "inputmode",
+  "accept",
+]);
+
+function isOmitWhenEmptyAttribute(name: string): boolean {
+  return OMIT_WHEN_EMPTY_ATTRIBUTES.has(name.toLowerCase());
+}
+
 const URL_ATTRIBUTES = new Set([
   "href",
   "src",
@@ -203,9 +226,11 @@ function renderAttr(attr: Attr): string {
     case "csrText":
       return "";
     default:
-      return attr.boolean
-        ? ` ${attr.name}`
-        : ` ${attr.name}="${attrEscape(safeAttributeValue(attr.name, attr.value))}"`;
+      if (attr.boolean) return ` ${attr.name}`;
+      // An empty constraint attribute is worse than absent -- see
+      // OMIT_WHEN_EMPTY_ATTRIBUTES.
+      if (attr.value === "" && isOmitWhenEmptyAttribute(attr.name)) return "";
+      return ` ${attr.name}="${attrEscape(safeAttributeValue(attr.name, attr.value))}"`;
   }
 }
 
@@ -2505,6 +2530,19 @@ function renderComponentNode(node: ViewNode, ctx: CompCtx): string {
         isExplicitComponentMount && wholeExpression
           ? `\${__wrnProp(${elementContext.resolveExpr(wholeExpression)})}`
           : compileAttrValue(a.value, elementContext);
+
+      /*
+       * A constraint attribute whose value is only known at render time (a
+       * component prop defaulting to "") must be decided at render time, so
+       * the whole attribute -- name included -- is emitted by the helper.
+       * Component mounts are excluded: there the value is a prop being passed
+       * down, not an HTML attribute being written.
+       */
+      if (isOmitWhenEmptyAttribute(a.name) && !isExplicitComponentMount) {
+        if (a.value === "") return "";
+        return `\${__wrnOptionalAttr(${JSON.stringify(a.name)}, \`${compiledValue}\`)}`;
+      }
+
       const rendered = ` ${a.name}="${compiledValue}"`;
 
       const referencesState = exprRefsComponentReactiveValue(a.value, ctx);
@@ -2880,6 +2918,10 @@ function __wrnHtml(v: unknown): string {
   );
 }
 
+function __wrnOptionalAttr(name: string, value: string): string {
+  return value === "" ? "" : " " + name + '="' + value + '"';
+}
+
 function __wrnAttr(v: unknown): string {
   return String(v == null ? "" : v).replace(
     /[&<>"]/g,
@@ -3046,6 +3088,10 @@ function __wrnHtml(v: unknown): string {
   );
 }
 
+function __wrnOptionalAttr(name: string, value: string): string {
+  return value === "" ? "" : " " + name + '="' + value + '"';
+}
+
 function __wrnAttr(v: unknown): string {
   return String(v == null ? "" : v).replace(/[&<>"]/g, (c) =>
     c === "&" ? "&amp;" : c === "<" ? "&lt;" : c === ">" ? "&gt;" : "&quot;",