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/main.tsx

@@ -5,8 +5,10 @@ import "./install";
 import "./styles.css";
 import { App } from "./App";
 import { applyTheme } from "./theme";
+import { captureShare } from "./share";
 
 applyTheme();
+captureShare();
 registerSW({ immediate: true });
 
 createRoot(document.getElementById("root")!).render(