feat(overlay): add primary subtitle bar visibility modes

- Cycle `v` through `hidden | visible | hover` instead of a boolean toggle
- Add `subtitleStyle.primaryDefaultMode` config with default `visible`
- Carry primary mode independently from secondary in hot-reload payload
- Add hover CSS: transparent until hovered, then fully visible
- Show primary-specific OSD text on each mode change

src/config/resolve/subtitle-domains.ts

@@ -147,6 +147,7 @@ export function applySubtitleDomainConfig(context: ResolveContext): void {
 
   if (isObject(src.subtitleStyle)) {
     const fallbackSubtitleStyleEnableJlpt = resolved.subtitleStyle.enableJlpt;
+    const fallbackSubtitleStylePrimaryDefaultMode = resolved.subtitleStyle.primaryDefaultMode;
     const fallbackSubtitleStylePreserveLineBreaks = resolved.subtitleStyle.preserveLineBreaks;
     const fallbackSubtitleStyleAutoPauseVideoOnHover = resolved.subtitleStyle.autoPauseVideoOnHover;
     const fallbackSubtitleStyleAutoPauseVideoOnYomitanPopup =
@@ -190,6 +191,24 @@ export function applySubtitleDomainConfig(context: ResolveContext): void {
       );
     }
 
+    const primaryDefaultMode = (src.subtitleStyle as { primaryDefaultMode?: unknown })
+      .primaryDefaultMode;
+    if (
+      primaryDefaultMode === 'hidden' ||
+      primaryDefaultMode === 'visible' ||
+      primaryDefaultMode === 'hover'
+    ) {
+      resolved.subtitleStyle.primaryDefaultMode = primaryDefaultMode;
+    } else if (primaryDefaultMode !== undefined) {
+      resolved.subtitleStyle.primaryDefaultMode = fallbackSubtitleStylePrimaryDefaultMode;
+      warn(
+        'subtitleStyle.primaryDefaultMode',
+        primaryDefaultMode,
+        resolved.subtitleStyle.primaryDefaultMode,
+        'Expected hidden, visible, or hover.',
+      );
+    }
+
     const preserveLineBreaks = asBoolean(
       (src.subtitleStyle as { preserveLineBreaks?: unknown }).preserveLineBreaks,
     );