ch output: per-channel output routing dropdowns

- Added GET/POST /api/channel-output endpoints for per-channel
  output routing (playback_1 / playback_2 / both)
- Added output dropdowns to CH 1 and CH 2 cards in Audio Interface
- Added setChOutput() and loadChOutput() JS functions
- Updated connect_fx_ports to read per-channel output config

src/web/server.py

@@ -997,6 +997,39 @@ class WebServer:
             })
             return {"ok": True, "channel_mode": mode}
 
+        # ── Per-channel output routing ────────────────────────────
+
+        @app.get("/api/channel-output")
+        async def get_channel_output():
+            """Get output routing for each channel."""
+            audio = self.deps.audio_system
+            if not audio or not audio.config:
+                return {"ch1": "both", "ch2": "both"}
+            cfg = audio.config.__dict__ if hasattr(audio.config, '__dict__') else {}
+            return {
+                "ch1": cfg.get("ch1_output", "both"),
+                "ch2": cfg.get("ch2_output", "both"),
+            }
+
+        @app.post("/api/channel-output")
+        async def set_channel_output(data: dict):
+            """Set output routing for a channel.
+
+            Body: { "ch": 1, "port": "playback_1"|"playback_2"|"both" }
+            """
+            ch = int(data.get("ch", 1))
+            port = data.get("port", "both")
+            if port not in ("playback_1", "playback_2", "both"):
+                raise HTTPException(status_code=400, detail=f"Invalid port: {port}")
+            audio = self.deps.audio_system
+            if audio and audio.config:
+                key = f"ch{ch}_output"
+                if hasattr(audio.config, 'input_device'):
+                    setattr(audio.config, key, port)
+                # Reconnect JACK ports
+                audio.connect_fx_ports()
+            return {"ok": True, f"ch{ch}": port}
+
         @app.get("/api/models")
         async def list_models(channel: str = "guitar"):
             """List available NAM models for the given channel."""