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/agent-form/agent-form-dropdowns.tsx

@@ -619,11 +619,6 @@ export function WorkingDirectoryDropdown({
         onPress={handleOpen}
         disabled={disabled}
         errorMessage={errorMessage || undefined}
-        helperText={
-          hasSuggestedPaths
-            ? "Search directories from existing agents or paste a new path."
-            : "No agent directories yet - search to add one."
-        }
         controlRef={anchorRef}
       />
       <AdaptiveSelect