Reduce unnecessary re-renders in agent panel and input components

Narrow zustand selectors to only subscribe to fields each component
actually uses. Split nested objects (availableModes, projectPlacement)
into separate selectors with structural equality checks. Remove the
updatedAt processing-spinner hack in agent-input-area — the server
already guarantees agent status is "running" before ACKing the send
request, so isProcessing can clear on submit success directly.

packages/app/src/components/agent-form/agent-form-dropdowns.tsx

@@ -537,14 +537,10 @@ export function AgentConfigRow({
   }, [modeOptions]);
 
   const modelSelectOptions: ComboSelectOption[] = useMemo(() => {
-    const opts: ComboSelectOption[] = [{ id: "", label: "Auto" }];
-    for (const model of models) {
-      opts.push({
-        id: model.id,
-        label: model.label,
-      });
-    }
-    return opts;
+    return models.map((model) => ({
+      id: model.id,
+      label: model.label,
+    }));
   }, [models]);
 
   const thinkingSelectOptions: ComboSelectOption[] = useMemo(
@@ -586,7 +582,7 @@ export function AgentConfigRow({
           title="Select model"
           value={selectedModel}
           options={modelSelectOptions}
-          placeholder="Auto"
+          placeholder={isModelLoading ? "Loading..." : "Select model"}
           disabled={disabled}
           isLoading={isModelLoading}
           onSelect={onSelectModel}
@@ -777,10 +773,8 @@ export function ModelDropdown({
   const [isOpen, setIsOpen] = useState(false);
   const anchorRef = useRef<View>(null);
 
-  const selectedLabel = selectedModel
-    ? (models.find((model) => model.id === selectedModel)?.label ?? selectedModel)
-    : "Automatic";
-  const placeholder = isLoading && models.length === 0 ? "Loading..." : "Automatic";
+  const selectedLabel = models.find((model) => model.id === selectedModel)?.label ?? selectedModel ?? "Select model";
+  const placeholder = isLoading && models.length === 0 ? "Loading..." : "Select model";
   const helperText = error
     ? undefined
     : isLoading
@@ -790,34 +784,20 @@ export function ModelDropdown({
         : undefined;
 
   const options = useMemo(() => {
-    const opts: ComboSelectOption[] = [
-      {
-        id: "",
-        label: "Automatic (provider default)",
-        description: "Let the assistant pick the recommended model.",
-      },
-    ];
-    for (const model of models) {
-      opts.push({
-        id: model.id,
-        label: model.label,
-        description: model.description,
-      });
-    }
-    return opts;
+    return models.map((model) => ({
+      id: model.id,
+      label: model.label,
+      description: model.description,
+    }));
   }, [models]);
 
   const handleOpen = useCallback(() => setIsOpen(true), []);
   const handleOpenChange = useCallback((open: boolean) => setIsOpen(open), []);
   const handleSelect = useCallback(
     (id: string) => {
-      if (id === "") {
-        onClear();
-      } else {
-        onSelect(id);
-      }
+      onSelect(id);
     },
-    [onClear, onSelect],
+    [onSelect],
   );
 
   return (