refactor: unify layout widths and font sizes

- Add MAX_CONTENT_WIDTH (820px) constant for consistent chat/form widths
- Bump theme font sizes (base now 16px) for better readability
- Apply max width to stream view, input area, and new agent form
- Position new agent form at bottom of scroll area
- Remove working directory helper text
- Add cmd+b sidebar toggle when textarea is focused

packages/app/src/components/message-input.tsx

@@ -28,6 +28,7 @@ import { useDictation } from "@/hooks/use-dictation";
 import { DictationOverlay } from "./dictation-controls";
 import type { UseWebSocketReturn } from "@/hooks/use-websocket";
 import type { SessionContextValue } from "@/contexts/session-context";
+import { useSidebarStore } from "@/stores/sidebar-store";
 
 export interface ImageAttachment {
   uri: string;
@@ -113,6 +114,7 @@ export const MessageInput = forwardRef<MessageInputRef, MessageInputProps>(
     ref
   ) {
     const { theme } = useUnistyles();
+    const toggleSidebar = useSidebarStore((state) => state.toggle);
     const [inputHeight, setInputHeight] = useState(MIN_INPUT_HEIGHT);
     const textInputRef = useRef<
       TextInput | (TextInput & { getNativeRef?: () => unknown }) | null
@@ -383,6 +385,13 @@ export const MessageInput = forwardRef<MessageInputRef, MessageInputProps>(
     if (!shouldHandleDesktopSubmit) return;
     const { shiftKey, metaKey, ctrlKey } = event.nativeEvent;
 
+    // Cmd+B or Ctrl+B: toggle sidebar
+    if ((metaKey || ctrlKey) && event.nativeEvent.key === "b") {
+      event.preventDefault();
+      toggleSidebar();
+      return;
+    }
+
     // Cmd+D or Ctrl+D: start dictation or submit if already dictating
     if ((metaKey || ctrlKey) && event.nativeEvent.key === "d") {
       event.preventDefault();