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/ChatView.tsx

@@ -1,3 +1,4 @@
+import { takePrefill } from "../share";
 import { Icon } from "./Icon";
 import { base64Size, FileChip, FileFigure, fileType } from "./FileFigure";
 import { useEffect, useLayoutEffect, useMemo, useRef, useState } from "react";
@@ -149,7 +150,7 @@ export function ChatView({ sessionId, onMenu }: { sessionId: string; onMenu: ()
   const [sharing, setSharing] = useState(false);
   const models = useModels();
   const [loadError, setLoadError] = useState<string>();
-  const [text, setText] = useState("");
+  const [text, setText] = useState(() => takePrefill(sessionId) ?? "");
   const [stats, setStats] = useState<SessionStats>();
   const [showStats, setShowStats] = useState(false);
   const [editingTitle, setEditingTitle] = useState(false);