[ACC-003] Revenue Circle selector missing aria-describedby for district dependency

[saqibmanan]

Summary

The Revenue Circle dropdown is disabled until a district is selected. Its placeholder reads "Select a district to enable" visually, but there is no aria-describedby linking the Revenue Circle input to the District selector. Screen readers have no programmatic way to understand the dependency.

Fix

<span id="rc-hint">Select a district first to enable this field</span>
<select aria-describedby="rc-hint" disabled>...</select>

[jpmckinney]

opub-ui's Select exposes only helpText, not arbitrary aria-describedby passthrough, so the fix requires opub-ui work.

[jpmckinney]

This is blocked until CivicDataLab/opub-mono#414 is merged.