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/Icon.tsx

@@ -193,6 +193,9 @@ const P: Record<string, ReactNode> = {
   ),
   dot: <circle cx="12" cy="12" r="4.5" className="fill" />,
   plus: <path d="M12 5v14M5 12h14" />,
+  pin: <path d="M9 4h6l-1 5 3 3v2H7v-2l3-3-1-5ZM12 14v6" />,
+  archive: <path d="M4 5h16v4H4ZM5 9v10h14V9M10 13h4" />,
+  template: <path d="M5 4h14a1 1 0 0 1 1 1v14a1 1 0 0 1-1 1H5a1 1 0 0 1-1-1V5a1 1 0 0 1 1-1ZM8 9h8M8 12.5h8M8 16h4.5" />,
   mic: (
     <>
       <rect x="9" y="3" width="6" height="11" rx="3" />