profiler editor: keep record/content fields visible

- Seed one empty record row when record content is created or reset
  (filetype change or Contents None->Exact/Mask) via profilerEmptyRecordContent
- Add per-record delete and '+ Record' add controls so the record view is
  always editable; delete is hidden on the last remaining row

SW cache v47.

frontend/tests/profiler.test.js

@@ -21,7 +21,7 @@ function extractFunc(src, name) {
 	return src.slice(m.index, i + 1);
 }
 
-const FNS = ['profilerNormHex', 'profilerMatch', 'profilerMatchMin', 'profilerMaskPrefix4', 'profilerFileFields', 'profilerContentKindForFileType', 'profilerValidateProfile'];
+const FNS = ['profilerNormHex', 'profilerMatch', 'profilerMatchMin', 'profilerMaskPrefix4', 'profilerFileFields', 'profilerContentKindForFileType', 'profilerEmptyRecordContent', 'profilerValidateProfile'];
 let code = '';
 for (const f of FNS) code += extractFunc(html, f) + '\n';
 eval(code);
@@ -92,6 +92,14 @@ test('profilerContentKindForFileType', () => {
 	assert.strictEqual(profilerContentKindForFileType('df'), 'transparent');
 });
 
+test('profilerEmptyRecordContent seeds one empty record row', () => {
+	const c = profilerEmptyRecordContent('exact');
+	assert.strictEqual(c.kind, 'record');
+	assert.strictEqual(c.mode, 'exact');
+	assert.strictEqual(c.records.length, 1);
+	assert.deepStrictEqual(c.records[0], { num: 1, data: '' });
+});
+
 test('profile validation', () => {
 	assert.strictEqual(profilerValidateProfile(null), 'Not an object');
 	assert.strictEqual(profilerValidateProfile({ name: 'x' }), 'Missing rules array');