Fix accessibility: message composer label and 'Seen by' button (#6390, #6393)

- MarkdownTextInput: set contentDescription on MarkdownEditText so TalkBack
  announces a persistent label even when the field has content (fixes #6390)
- TimelineItemReadReceiptView: remove hideFromAccessibility() so the 'Seen by'
  button is exposed to TalkBack; use semantics (not clearAndSetSemantics) to
  preserve click action, and add Role.Button (fixes #6393)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

libraries/textcomposer/impl/src/main/kotlin/io/element/android/libraries/textcomposer/components/markdown/MarkdownTextInput.kt

@@ -93,6 +93,7 @@ fun MarkdownTextInput(
                 setText(text)
                 setHint(placeholder)
                 setHintTextColor(ColorStateList.valueOf(placeholderColor.toArgb()))
+                contentDescription = placeholder
                 inputType = InputType.TYPE_CLASS_TEXT or
                     InputType.TYPE_TEXT_FLAG_CAP_SENTENCES or
                     InputType.TYPE_TEXT_FLAG_MULTI_LINE or
@@ -129,6 +130,7 @@ fun MarkdownTextInput(
             }
         },
         update = { editText ->
+            editText.contentDescription = placeholder
             editText.applyStyleInCompose(richTextEditorStyle)
             val text = state.text.value()
             mentionSpanUpdater.updateMentionSpans(text)