Harden theme picker: null-safe widget cast + accessibility

Address CodeRabbit findings:
- Use safe cast (as?) with early return for the widget_frame lookup.
- Add contentDescription to modern list rows (theme name) and classic
  grid swatches (Color theme %d) for screen-reader support.

Author: hawkff

app/src/main/java/moe/matsuri/nb4a/ui/ThemePickerPreference.kt

@@ -50,7 +50,8 @@ class ThemePickerPreference
     override fun onBindViewHolder(holder: PreferenceViewHolder) {
         super.onBindViewHolder(holder)
 
-        val widgetFrame = holder.findViewById(android.R.id.widget_frame) as LinearLayout
+        val widgetFrame = holder.findViewById(android.R.id.widget_frame) as? LinearLayout
+            ?: return
 
         if (!inited) {
             inited = true
@@ -147,6 +148,8 @@ class ThemePickerPreference
                 android.R.attr.selectableItemBackground, outValue, true
             )
             setBackgroundResource(outValue.resourceId)
+            // Announce the theme name to screen readers when the row is focused.
+            contentDescription = name
 
             val leading = when {
                 swatchColor != null -> nekoImageView(swatchColor, 28, 0)
@@ -195,6 +198,8 @@ class ThemePickerPreference
                 if (Theme.MODERN_THEMES.any { it.id == themeId }) continue
 
                 addView(nekoImageView(color, 64, 0).apply {
+                    contentDescription =
+                        context.getString(R.string.theme_classic_color_swatch, themeId)
                     setOnClickListener {
                         select(themeId)
                         dialog.dismiss()

app/src/main/res/values/strings.xml

@@ -16,6 +16,7 @@
     <string name="theme">Theme</string>
     <string name="theme_dynamic">Dynamic (Material You)</string>
     <string name="theme_classic_colors">Classic colors…</string>
+    <string name="theme_classic_color_swatch">Color theme %1$d</string>
     <string name="document">Document</string>
     <string name="group_default">Ungrouped</string>
     <string name="quick_toggle">Toggle</string>