fix(ui): provide explicit undefined defaults for optional props

Button.onclick, Input.id, Select.id were declared without defaults
(implicitly undefined) which could cause Svelte 5 warnings or incorrect
() destructuring behavior. Set explicit = undefined.

frontend/src/lib/ui/Button.svelte

@@ -33,7 +33,7 @@
     type = /** @type {'button' | 'submit' | 'reset'} */ ('button'),
     class: className = '',
     children,
-    onclick,
+    onclick = undefined,
     ...rest
   } = $props();
   // [/SECTION: PROPS]

frontend/src/lib/ui/Input.svelte

@@ -30,7 +30,7 @@
     error = "",
     disabled = false,
     type = "text",
-    id: externalId,
+    id: externalId = undefined,
     class: className = "",
     ...rest
   } = $props();

frontend/src/lib/ui/Select.svelte

@@ -25,7 +25,7 @@
     value = $bindable(""),
     options = [],
     disabled = false,
-    id: externalId,
+    id: externalId = undefined,
     class: className = "",
     ...rest
   } = $props();