Unify backup/restore into single v2.0 file with full state capture

Server export now includes encryption key, themes, and all config files.
Client export bundles all DashCaddy localStorage keys (19 named + dynamic
widget keys) as browserState. Restore handles both server and browser
state in one operation. Legacy v1.0 import format still supported.
Removed redundant Export/Import toolbar buttons — Backup modal is now
the single entry point.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

dashcaddy-api/crypto-utils.js

@@ -315,6 +315,14 @@ function decryptWithKey(encryptedData, key) {
 // Initialize key on module load
 loadOrCreateKey();
 
+/**
+ * Clear the cached encryption key so it reloads from file on next use.
+ * Called after restoring an encryption key from backup.
+ */
+function clearCachedKey() {
+  encryptionKey = null;
+}
+
 module.exports = {
   encrypt,
   decrypt,
@@ -327,5 +335,6 @@ module.exports = {
   loadOrCreateKey,
   deriveKey,
   rotateKey,
-  decryptWithKey
+  decryptWithKey,
+  clearCachedKey
 };