feat: auto-save and restore session state (#135)

- Save and restore chat messages, XML snapshots, session ID, and diagram XML to localStorage
- Restore diagram when DrawIO becomes ready (using new onLoad callback)
- Change close protection default to false since auto-save handles persistence
- Clear localStorage when clearing chat
- Handle edge cases: undefined edit fields, empty chartXML, missing access code header

lib/utils.ts

@@ -467,7 +467,9 @@ export function replaceXMLParts(
                     i,
                     i + searchLines.length,
                 )
-                const normalizedCandidate = normalizeWs(candidateLines.join(" "))
+                const normalizedCandidate = normalizeWs(
+                    candidateLines.join(" "),
+                )
 
                 if (normalizedCandidate === normalizedSearch) {
                     matchStartLine = i