Add programmatic label to Simplified Summary textarea

The Simplified Summary textareas in the editor sidebar relied solely on
placeholder text for their accessible name, which disappears once the
user types. Add a visually hidden label so assistive technology always
announces the field's purpose (WCAG 3.3.2).

Fixes #1766

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016H6Hjd5LrCUVG8LYiQ6y7X

Author: claude

src/sidebar/components/Panels/ReadabilityAnalysis.js

@@ -362,6 +362,8 @@ const ReadabilityAnalysis = () => {
 										{ __( 'Simplified Summary', 'accessibility-checker' ) }
 									</h4>
 									<TextareaControl
+										label={ __( 'Simplified Summary', 'accessibility-checker' ) }
+										hideLabelFromVision={ true }
 										value={ summaryText }
 										onChange={ setSummaryText }
 										placeholder={ __( 'Enter simplified summary...', 'accessibility-checker' ) }
@@ -403,6 +405,8 @@ const ReadabilityAnalysis = () => {
 								{ __( 'Simplified Summary', 'accessibility-checker' ) }
 							</h4>
 							<TextareaControl
+								label={ __( 'Simplified Summary', 'accessibility-checker' ) }
+								hideLabelFromVision={ true }
 								value={ summaryText }
 								onChange={ setSummaryText }
 								placeholder={ __( 'Enter simplified summary...', 'accessibility-checker' ) }