fix: honor false defaults on checkbox prompt variables (#30037)

A prompt template with `{{flag | checkbox:default=false}}` opened the input variables form with the checkbox already ticked, and the same happened for `False`, `"false"`, `0` and `"0"`. Template defaults arrive as strings and the checkbox was bound straight to that value, so any non-empty string counted as checked and there was no way to define a checkbox that starts unchecked once a default was given.

The checkbox now reads its ticked state from the value explicitly: it is checked only for `true` (boolean or string) and stays unchecked for anything else, while clicking it still writes a boolean like before. The seeded value itself is left alone, so what gets substituted into the prompt for an untouched form is unchanged, and typing `true` or `false` into the text field next to the checkbox now moves the tick accordingly.

Fixes #30036

src/lib/components/chat/MessageInput/InputVariablesModal.svelte

@@ -140,7 +140,9 @@
 														<div class="relative flex justify-center items-center gap-2">
 															<input
 																type="checkbox"
-																bind:checked={variableValues[variable]}
+																checked={variableValues[variable] === true ||
+																	variableValues[variable] === 'true'}
+																on:change={(e) => (variableValues[variable] = e.target.checked)}
 																class="size-3.5 rounded cursor-pointer border border-gray-200 dark:border-gray-700"
 																id="input-variable-{idx}"
 																{...variableAttributes}