feat(overlay): add primary subtitle bar visibility modes (#63)

- 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/definitions/options-subtitle.ts

@@ -5,6 +5,14 @@ export function buildSubtitleConfigOptionRegistry(
   defaultConfig: ResolvedConfig,
 ): ConfigOptionRegistryEntry[] {
   return [
+    {
+      path: 'subtitleStyle.primaryDefaultMode',
+      kind: 'enum',
+      enumValues: ['hidden', 'visible', 'hover'],
+      defaultValue: defaultConfig.subtitleStyle.primaryDefaultMode,
+      description:
+        'Default primary subtitle bar visibility mode. hidden hides it, visible shows it, hover reveals it on hover.',
+    },
     {
       path: 'subtitleStyle.enableJlpt',
       kind: 'boolean',