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/defaults-subtitle.ts

@@ -2,6 +2,7 @@ import { ResolvedConfig } from '../../types/config';
 
 export const SUBTITLE_DEFAULT_CONFIG: Pick<ResolvedConfig, 'subtitleStyle' | 'subtitleSidebar'> = {
   subtitleStyle: {
+    primaryDefaultMode: 'visible',
     enableJlpt: false,
     preserveLineBreaks: false,
     autoPauseVideoOnHover: true,