🤖 fix: restore vertical token meter on mobile collapsed state (#525)

Fixes the strange rectangle that appeared at the bottom of the screen on
mobile viewports when the right sidebar was collapsed.

## Problem

On mobile (max-width: 768px), the collapsed right sidebar used
`position: absolute; bottom: 0; width: 0` which created a visible
rectangular artifact at the bottom of the viewport.

This bug was introduced in commit 4d7ba60 (Oct 21, 2025) during the
initial mobile responsive implementation and has been present since
then.

## Solution

Removed the mobile-specific override that forced collapsed sidebar to
`width: 0` with absolute positioning at the bottom. Now mobile behavior
matches desktop:

**Before:**
- Collapsed: `width: 0; position: absolute; bottom: 0` → Strange visible
rectangle
- Expanded: Full width, 50vh max height

**After:**
- Collapsed: 20px vertical token meter bar (same as desktop)
- Expanded: Full width, 50vh max height

The vertical token meter provides useful context about token usage and
this approach matches user expectations from the desktop experience.

## Testing

Verify at various viewport widths:
- Desktop (>768px): Vertical meter shows when collapsed ✓
- Tablet/Mobile (≤768px): Vertical meter shows when collapsed (no more
weird rectangle) ✓
- Mobile expanded: Full width panel at bottom with proper max-height ✓

_Generated with `cmux`_

Author: ammar-agent

src/components/RightSidebar.tsx

@@ -52,8 +52,8 @@ const SidebarContainer: React.FC<SidebarContainerProps> = ({
         "bg-separator border-l border-border-light flex flex-col overflow-hidden flex-shrink-0",
         customWidth ? "" : "transition-[width] duration-200",
         collapsed && "sticky right-0 z-10 shadow-[-2px_0_4px_rgba(0,0,0,0.2)]",
+        // Mobile: Show vertical meter when collapsed (20px), full width when expanded
         "max-md:border-l-0 max-md:border-t max-md:border-border-light",
-        collapsed && "max-md:w-0 max-md:absolute max-md:bottom-0",
         !collapsed && "max-md:w-full max-md:relative max-md:max-h-[50vh]"
       )}
       style={{ width }}