First upload, 18 controller version

app/output/preview.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+from app.config.models import InfinityMirrorConfig
+from app.core.types import PreviewFrame
+
+from .base import OutputBackend, OutputResult
+
+
+class PreviewOutputBackend(OutputBackend):
+    backend_id = "preview"
+    display_name = "Preview Only"
+    supports_live_output = False
+
+    def send_frame(self, config: InfinityMirrorConfig, frame: PreviewFrame) -> OutputResult:
+        return OutputResult(ok=True, message="Preview-only mode active.", packets_sent=0, device_count=0)