webui: export conversations as jsonl (#24688)

* webui: export conversations as jsonl

each session is one jsonl file, a session header line followed by one line per message
exporting multiple conversations bundles them into a zip, one jsonl file each

* webui: import jsonl and zip conversation exports

parse the new jsonl session format and zip archives on import
keep supporting the legacy json format

tools/ui/package.json

@@ -59,6 +59,7 @@
 		"eslint-config-prettier": "10.1.8",
 		"eslint-plugin-storybook": "10.4.2",
 		"eslint-plugin-svelte": "3.19.0",
+		"fflate": "0.8.3",
 		"globals": "16.5.0",
 		"highlight.js": "11.11.1",
 		"http-server": "14.1.1",