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.tsx

@@ -56,8 +56,8 @@ const userMessageStylesheet = StyleSheet.create((theme) => ({
   },
   text: {
     color: theme.colors.foreground,
-    fontSize: theme.fontSize.lg,
-    lineHeight: 24,
+    fontSize: theme.fontSize.base,
+    lineHeight: 22,
   },
   bubblePressed: {
     opacity: 0.85,