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/main/runtime/config-hot-reload-handlers.ts

@@ -54,6 +54,7 @@ export function buildConfigHotReloadPayload(config: ResolvedConfig): ConfigHotRe
     sessionBindingWarnings,
     subtitleStyle: resolveSubtitleStyleForRenderer(config),
     subtitleSidebar: config.subtitleSidebar,
+    primarySubMode: config.subtitleStyle.primaryDefaultMode,
     secondarySubMode: config.secondarySub.defaultMode,
   };
 }