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/renderer/style.css

@@ -684,6 +684,16 @@ body.subtitle-sidebar-embedded-open #subtitleContainer {
   display: none;
 }
 
+#subtitleContainer.primary-sub-hover {
+  opacity: 0;
+  transition: opacity 0.2s ease;
+  pointer-events: auto;
+}
+
+#subtitleContainer.primary-sub-hover:hover {
+  opacity: 1;
+}
+
 #subtitleContainer.primary-sub-hidden {
   display: none;
   pointer-events: none;