feat(immersive): make window draggable and rename to Immersive Mode

- Add persistent drag region (32px strip) at top of immersive overlay
  that remains active even when the header auto-hides after 4 seconds
- Add app-region: drag CSS to ImmersiveHeader's left spacer for proper
  drag support when header is visible
- Add app-region: no-drag to tabs and window controls to keep them
  clickable
- Double-click on drag strip toggles maximize
- Rename "Full Screen" to "Immersive Mode" in all 4 locales (en/es/de/fr)

src/lib/i18n/locales/fr.json

@@ -442,8 +442,8 @@
     "castToDevice": "Caster vers un appareil",
     "castingManage": "Casting - Cliquez pour gérer",
     "miniPlayer": "Mini lecteur",
-    "fullScreen": "Plein écran",
-    "exitFullScreen": "Quitter le plein écran",
+    "fullScreen": "Mode Immersif",
+    "exitFullScreen": "Quitter le Mode Immersif",
     "maximize": "Agrandir",
     "restore": "Restaurer",
     "minimize": "Réduire",