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/types/subtitle.ts

@@ -55,7 +55,9 @@ export interface SubtitleStyle {
   fontSize: number;
 }
 
-export type SecondarySubMode = 'hidden' | 'visible' | 'hover';
+export type SubtitleBarMode = 'hidden' | 'visible' | 'hover';
+export type PrimarySubMode = SubtitleBarMode;
+export type SecondarySubMode = SubtitleBarMode;
 
 export interface SecondarySubConfig {
   secondarySubLanguages?: string[];
@@ -67,6 +69,7 @@ export type NPlusOneMatchMode = 'headword' | 'surface';
 export type FrequencyDictionaryMatchMode = 'headword' | 'surface';
 
 export interface SubtitleStyleConfig {
+  primaryDefaultMode?: PrimarySubMode;
   enableJlpt?: boolean;
   preserveLineBreaks?: boolean;
   autoPauseVideoOnHover?: boolean;