Remove FAST/THINKING model dropdown from chat composer (#106) (#107)

## Summary
- Closes [#106](#106).
- Removes the FAST / THINKING `<select>` from the chat input bar.
- Continues to send a fixed `modelName` of `FAST` on WebSocket sends so the backend contract is unchanged.

## Test plan
- [ ] Open chat dashboard — composer shows attach + text field + send only (no FAST/THINKING dropdown).
- [ ] Send a message — stream still works.
- [ ] Send with attachment — still works.Reviewed-on: #107

llm-fe/src/llm-fe/pages/AsyncDashboard2/AsyncDashboard2.tsx

@@ -252,32 +252,6 @@ const IconButton = styled.button`
   }
 `;
 
-const StyledSelect = styled.select`
-  background: transparent;
-  border: none;
-  flex-shrink: 0;
-  max-width: 7rem;
-  color: ${({ theme }) => theme.colors.text};
-  font-size: 0.9rem;
-  padding: 0.5rem;
-  outline: none;
-  cursor: pointer;
-  margin-right: 0.5rem;
-  border-right: 1px solid ${({ theme }) => theme.colors.cardBorder};
-
-  @media (max-width: 768px) {
-    max-width: 5.5rem;
-    font-size: 0.8rem;
-    padding: 0.4rem 0.2rem;
-    margin-right: 0.25rem;
-  }
-  
-  option {
-    background: ${({ theme }) => theme.colors.background || '#1a1a1a'};
-    color: ${({ theme }) => theme.colors.text};
-  }
-`;
-
 const ConversationItem = styled.div<{ $active: boolean }>`
   padding: 0.8rem 1rem;
   margin-bottom: 0.5rem;
@@ -343,11 +317,13 @@ const validationSchema = Yup.object().shape({
     .required("This is required"),
 });
 
+/** Fixed until product re-exposes a model picker (#106). */
+const DEFAULT_MODEL_NAME = "FAST";
+
 type PromptValues = {
   prompt: string;
   file: Blob | null;
   fileType: string | null;
-  modelName: string;
 };
 
 const AlwaysScrollToBottom = ({ trigger }: { trigger: string | number }): JSX.Element => {
@@ -447,7 +423,7 @@ const AsyncDashboardInner = (): JSX.Element => {
   })();
 
   const handlePromptSubmit = async (
-    { prompt, file, fileType, modelName }: PromptValues,
+    { prompt, file, fileType }: PromptValues,
     { resetForm }: any,
   ): Promise<void> => {
     const trimmedPrompt = (prompt ?? "").trim();
@@ -464,7 +440,7 @@ const AsyncDashboardInner = (): JSX.Element => {
 
       conversationRef.current = tempConversations;
       setConversationDetails(tempConversations);
-      sendMessage(trimmedPrompt, selectedConversation, file, fileType, modelName);
+      sendMessage(trimmedPrompt, selectedConversation, file, fileType, DEFAULT_MODEL_NAME);
       trackEvent(AnalyticsEvents.MESSAGE_SENT, {
         hasConversation: Boolean(selectedConversation),
         hasAttachment: Boolean(file),
@@ -474,7 +450,6 @@ const AsyncDashboardInner = (): JSX.Element => {
           prompt: "",
           file: null,
           fileType: null,
-          modelName: modelName, // Keep the selected model
         }
       });
 
@@ -640,7 +615,6 @@ const AsyncDashboardInner = (): JSX.Element => {
               prompt: "",
               file: null,
               fileType: null,
-              modelName: "FAST",
             }}
             validationSchema={validationSchema}
             onSubmit={handlePromptSubmit}
@@ -691,14 +665,6 @@ const AsyncDashboardInner = (): JSX.Element => {
                     }}
                   />
 
-                  <Field
-                    as={StyledSelect}
-                    name="modelName"
-                  >
-                    <option value="FAST">FAST</option>
-                    <option value="THINKING">THINKING</option>
-                  </Field>
-
                   <Field name="prompt">
                     {({ field }: any) => (
                       <StyledInput