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>

web/src/components/NewSession.tsx

@@ -7,6 +7,7 @@ import type { ImageData } from "../types";
 import { readImage } from "./Composer";
 import { formatSize } from "./FileFigure";
 import { ModelSelect } from "./ModelSelect";
+import { takePrefill } from "../share";
 
 interface DirListing {
   path: string;
@@ -158,11 +159,13 @@ export function NewSession({ onMenu, onCreated }: { onMenu: () => void; onCreate
   const t = useT();
   const roots = useFetch<{ path: string; name: string }[]>("/fs/roots").data ?? [];
   const config = useFetch<{ default_model?: string }>("/config").data;
-  const [task, setTask] = useState<Task>("craft");
+  // Text shared from another app (PWA share target) starts a normal task.
+  const [prefill] = useState(() => takePrefill("new"));
+  const [task, setTask] = useState<Task>(prefill ? "build" : "craft");
   const [target, setTarget] = useState<Target>("fresh");
   const [deep, setDeep] = useState(false);
   const [url, setUrl] = useState("");
-  const [text, setText] = useState("");
+  const [text, setText] = useState(prefill ?? "");
   const [images, setImages] = useState<ImageData[]>([]);
   const [files, setFiles] = useState<File[]>([]);
   const [model, setModel] = useState("");