luci-base: fix label association warning for Dropdown widget

[stangri]

Summary

• Add data-widget-id attribute to the Dropdown widget's container <div>, matching the pattern already used by Checkbox (ui.js line 647)
• This allows renderFrame()'s existing fallback selector (form.js line 4454) to find the widget, resolving browser console accessibility warnings

Problem

The Dropdown widget renders as a <div>, which is not a labelable HTML element. When used with MultiValue or other dropdown-based form options, renderFrame() creates a <label for="widget.cbid..."> but no element with that id or data-widget-id exists in the Dropdown output.

This causes browser DevTools to report:

• "A <label> element with a for attribute doesn't match any element id"

Fix

Single-line addition — add data-widget-id to UIDropdown.render():

'data-widget-id': this.options.id ? `widget.${this.options.id}` : null,

This leverages the existing fallback mechanism in renderFrame():

const elem = node.nextElementSibling.querySelector(`#${node.getAttribute('for')}`)
    ?? node.nextElementSibling.querySelector(`[data-widget-id="${node.getAttribute('for')}"]`);

Test plan

• Open any LuCI page using MultiValue widget (e.g., any app with a multi-select dropdown)
• Check browser DevTools console — label association warning should no longer appear
• Verify clicking the label text still focuses/opens the dropdown

🤖 Generated with Claude Code

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[systemcrash]

A bit of a can of worms here. I can only reproduce this on Chrome which generates those notices, and I cannot seem to silence the notice with this change alone. The checkboxes which use 'label' for the text label (also) generate the notice.