feat: implement new session behavior to clear selection and show empty state

- Added bilingual notes for the new session feature, detailing the transition to an empty state upon session creation.
- Updated `SessionsService` to include a `clear()` method that resets the current selection and persists the empty state.
- Enhanced the `EmptyState` component to reflect the new design, including workspace selection and input handling.
- Modified CSS styles for improved layout and visual consistency in the empty state.
- Updated tests to cover the new session clearing functionality and its effects on the UI.

packages/client/ui-sidebar/src/client/contract/slots.ts

@@ -25,8 +25,8 @@ export type SidebarRootInjected = {
   /** Open (switch to) a session. */
   onOpen: (id: SessionId) => void
   /**
-   * Create a session and open it; cwd targets a project group (the
-   * sidebar's three creation entries all land in the new session).
+   * New-session affordance: no cwd clears selection onto the empty-state
+   * launch; a cwd create-then-opens a session in that project group.
    */
   onCreate: (cwd?: string) => void
   /** Collapse the sidebar column (layout service action; owner share stays {collapsed,width}). */

packages/client/ui-sidebar/src/client/index.ts

@@ -27,9 +27,15 @@ export function apply(ctx: ClientContext): void {
     // list snapshot); layout keeps only panel geometry.
     onOpen: (id) => { ctx.sessions.open(id) },
     onCreate: (cwd) => {
-      // Create-then-open: the sidebar's three creation entries all land
-      // in the new session (empty-state first-send stays with ui-conversation).
-      void ctx.sessions.create(cwd === undefined ? {} : { cwd })
+      // Top-level New Session / New Workspace: clear selection so AppFrame
+      // shows conversation.empty (EmptyState + shared InputBar). Per-project
+      // "+" still create-then-opens into that cwd until workspace seeding
+      // reaches the empty-state picker.
+      if (cwd === undefined) {
+        ctx.sessions.clear()
+        return
+      }
+      void ctx.sessions.create({ cwd })
         .then((id: SessionId) => { ctx.sessions.open(id) })
     },
     onToggleSidebar: () => { ctx.layout.toggleSidebar() },