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/src/client/SidebarRoot.module.css

@@ -79,13 +79,19 @@
 
 /* Brand group (figma I133:7632): the full wordmark rides the text ink
    (figma-flows ruling: main-screen instance is black; blue is brand
-   emphasis only). */
+   emphasis only). A button only in behavior (New Session shortcut): the
+   pointer cursor is the sole affordance — no hover chrome on the mark. */
 .brand {
   flex: 1;
   min-width: 0;
   display: inline-flex;
   align-items: center;
   overflow: hidden;
+  padding: 0;
+  border: none;
+  background: transparent;
+  color: inherit;
+  cursor: pointer;
 }
 
 .iconButton {

packages/client/ui-sidebar/src/client/SidebarRoot.tsx

@@ -61,10 +61,17 @@ export function SidebarRoot({
       style={wide ? { width: collapsed ? lastWideWidth.current : width } : undefined}
     >
       <div className={css.logoRow}>
+        {/* Expanded, the wordmark doubles as a New Session shortcut; the
+            collapsed rail's logo is the expand toggle below instead. */}
         {wide && (
-          <span className={clsx(css.brand, css.wide)}>
+          <button
+            type="button"
+            className={clsx(css.brand, css.wide)}
+            aria-label="New session"
+            onClick={() => { startSession() }}
+          >
             <BrandWordmark />
-          </span>
+          </button>
         )}
         {/* Rail resting state is the whale mark; hovering swaps in the panel
             icon (the expand affordance, figma sidebar-hover flow). */}