Add share target, prompt templates, command palette and session organisation

- PWA share target: shared text/links start a new chat or go into a
  recent session (chooser dialog)
- Prompt templates (personal + global for admins) with {{placeholders}}
  that are asked for on insert; managed under Settings → Templates
- Ctrl/Cmd+K command palette; Alt+N new session, Alt+Up/Down switch
  sessions, Ctrl+, settings
- Sessions can be pinned, archived and filed into folders; the list can
  be grouped by date or by project, groups are collapsible
- Fix: the "What's new" dialog was never shown to signed-in users

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/src/runner.rs

@@ -700,6 +700,9 @@ fn create_child(app: &Arc<App>, ctx: &ToolCtx, agent_name: &str, description: &s
         clean_room: parent.clean_room,
         phase: None,
         worktree: None,
+        pinned: false,
+        archived: false,
+        folder: None,
     };
     let child = app.save_session(child)?;
     let msg = Message::new(Role::User, vec![Part::Text { text: prompt }]);