feat: add Progressive Web App (PWA) support with service worker registration, manifest, update notifications, and install prompts

backend/main.py

@@ -1532,6 +1532,33 @@ async def api_diagnostics(current_user=Depends(require_admin)):
 if FRONTEND_DIR.exists():
     app.mount("/static", StaticFiles(directory=str(FRONTEND_DIR)), name="static")
 
+    @app.get("/sw.js")
+    async def serve_service_worker():
+        """Serve the service worker for PWA support."""
+        sw_file = FRONTEND_DIR / "sw.js"
+        if sw_file.exists():
+            return FileResponse(
+                sw_file,
+                media_type="application/javascript",
+                headers={
+                    "Cache-Control": "no-cache, no-store, must-revalidate",
+                    "Service-Worker-Allowed": "/"
+                }
+            )
+        raise HTTPException(status_code=404, detail="Service worker not found")
+
+    @app.get("/manifest.json")
+    async def serve_manifest():
+        """Serve the PWA manifest."""
+        manifest_file = FRONTEND_DIR / "manifest.json"
+        if manifest_file.exists():
+            return FileResponse(
+                manifest_file,
+                media_type="application/manifest+json",
+                headers={"Cache-Control": "public, max-age=3600"}
+            )
+        raise HTTPException(status_code=404, detail="Manifest not found")
+
     @app.get("/popout/{vault_name}/{path:path}")
     async def serve_popout(vault_name: str, path: str):
         """Serve the minimalist popout page for a specific file."""