Add save-to-device export, server export route and an On other devices P2P list

frontend/export.test.js

@@ -0,0 +1,10 @@
+const test = require('node:test');
+const assert = require('node:assert');
+const { fileName } = require('./export.js');
+
+test('fileName strips path and reserved characters and keeps the extension', () => {
+  assert.strictEqual(fileName('AC/DC: Back "In" Black?', 'mp4'), 'AC DC Back In Black .mp4'.replace(' .', '.'));
+  assert.strictEqual(fileName('  ', '.M4A', 'abc123'), 'abc123.m4a');
+  assert.strictEqual(fileName('x'.repeat(300), 'mp4').length, 124);
+  assert.strictEqual(fileName('Song', '../../x'), 'Song.x');
+});