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/vite.config.ts

@@ -29,6 +29,8 @@ export default defineConfig({
         scope: "/",
         orientation: "any",
         categories: ["developer", "productivity"],
+        // Android "Share" sheet: text/links become a new chat or go into a recent one.
+        share_target: { action: "/", method: "GET", params: { title: "share_title", text: "share_text", url: "share_url" } },
         icons: [
           { src: "icon-192.png", sizes: "192x192", type: "image/png" },
           { src: "icon-512.png", sizes: "512x512", type: "image/png" },