fix(web-ui): workspace chip placeholder, logo new-session shortcut, hero foot padding

- The hero workspace chip is a selector: no-live-selection states (cold
  start, workspace deleted from the sidebar after the list is ready) now
  render a "Choose workspace" placeholder (closed-folder icon) instead of
  resurrecting the deleted folder name via the session cwd; the cwd-derived
  name still bridges the initial list load. Stale pending picks clear when
  their workspace leaves a ready list.
- The expanded sidebar wordmark starts a new session (visuals unchanged,
  pointer cursor only); the collapsed rail logo keeps its expand toggle.
- The centered hero composer stack gains a 32px foot for visual balance.

packages/client/ui-sidebar/tests/sidebar-root.spec.tsx

@@ -54,10 +54,13 @@ function mountShell({ collapsed = false, width = 300 }: { collapsed?: boolean; w
 }
 
 describe('SidebarRoot shell', () => {
-  it('routes New Session and the column toggle', () => {
+  it('routes New Session (capsule + wordmark) and the column toggle', () => {
     const b = mountShell()
-    fireEvent.click(screen.getByRole('button', { name: 'New session' }))
-    expect(b.startSession).toHaveBeenCalledWith()
+    // Expanded, both the wordmark and the capsule start a session.
+    const starters = screen.getAllByRole('button', { name: 'New session' })
+    expect(starters).toHaveLength(2)
+    for (const button of starters) fireEvent.click(button)
+    expect(b.startSession).toHaveBeenCalledTimes(2)
     fireEvent.click(screen.getByRole('button', { name: 'Collapse sidebar' }))
     expect(b.toggleSidebar).toHaveBeenCalledOnce()
   })