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-lock.json

@@ -40,6 +40,7 @@
 				"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",
@@ -9454,6 +9455,13 @@
 				}
 			}
 		},
+		"node_modules/fflate": {
+			"version": "0.8.3",
+			"resolved": "https://registry.npmjs.org/fflate/-/fflate-0.8.3.tgz",
+			"integrity": "sha512-tbZNuJrLwGUp3zshBtdy4W+ORxZuIh8a5ilyIEQDC5rY1f3U20JMry0Ll3WBzU58EZKsEuJFXhb5gwv8CsPvgA==",
+			"dev": true,
+			"license": "MIT"
+		},
 		"node_modules/file-entry-cache": {
 			"version": "8.0.0",
 			"resolved": "https://registry.npmjs.org/file-entry-cache/-/file-entry-cache-8.0.0.tgz",