Fix multiple checkbox values lost in GET form submissions

[Copilot]

When submitting forms with multiple checkboxes sharing the same name via GET, only the last value was preserved in query parameters instead of all selected values (?name=value3 instead of ?name=value1&name=value2&name=value3). This caused server-side parameter parsing failures.

Changes

• helpers.js line 199: Changed url.searchParams.set() to url.searchParams.append() to match the correct behavior already present on line 281

The issue was an inconsistency in query parameter building - the code that constructs the actual server request correctly used append(), but the code that updates the browser address bar used set(), which overwrites duplicate keys.

// Before (line 199)
url.searchParams.set(pair[0], pair[1]);  // Overwrites previous values

// After (line 199)
url.searchParams.append(pair[0], pair[1]);  // Preserves all values

This ensures both the displayed URL and the actual request contain all checkbox values.

Original prompt

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This section details on the original issue you should resolve

<issue_title>helper.js does not submit multiple checkbox values via query parameter</issue_title>
<issue_description>When submitting a form with multiple checkbox values of the same group using the GET method, the browser is suppose to repeat the checkbox name with each value in the query parameter:

?name=value1&name=value2&name=value3

However, with helper.js enabled, only the last value gets appended to the query parameters:

?name=value3

This interferes with the server-side parameters parsing and will result in a NoConstructorMatched error.

Removing helper.js from Web/View/Layout.hs fixes this issue.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes helper.js does not submit multiple checkbox values via query parameter #1824

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