refactor: move GenAI parameters to Chat tab, simplify GenAI Config

Remove temperature, max_tokens, top_p, top_k, frequency/presence
penalty fields from the GenAI Config tab. That tab is now focused
on adding custom models not in the catalog.

Parameters are now managed exclusively in the Chat tab via the
collapsible gear panel, available for both direct models and saved
configs. Inline params override stored defaults when using presets.

backend/app.py

@@ -1577,7 +1577,16 @@ async def chat(msg: ChatMsg, u=Depends(current_user)):
     genai_cfg = None
     if msg.genai_config_id:
         with db() as c:
-            genai_cfg = c.execute("SELECT * FROM genai_configs WHERE id=?", (msg.genai_config_id,)).fetchone()
+            row = c.execute("SELECT * FROM genai_configs WHERE id=?", (msg.genai_config_id,)).fetchone()
+        if row:
+            genai_cfg = dict(row)
+            # Override params from inline chat settings if provided
+            if msg.temperature is not None: genai_cfg["temperature"] = msg.temperature
+            if msg.max_tokens is not None: genai_cfg["max_tokens"] = msg.max_tokens
+            if msg.top_p is not None: genai_cfg["top_p"] = msg.top_p
+            if msg.top_k is not None: genai_cfg["top_k"] = msg.top_k
+            if msg.frequency_penalty is not None: genai_cfg["frequency_penalty"] = msg.frequency_penalty
+            if msg.presence_penalty is not None: genai_cfg["presence_penalty"] = msg.presence_penalty
     elif msg.model_id and msg.oci_config_id:
         # Direct model mode: build synthetic config dict
         with db() as c: