fix: show Laya model download progress and surface errors in Settings (#12)

Track download progress in layaService (bytes/total, phase, error
state) and expose it through the laya status route. Render a progress
bar during model download in Settings via a layaStatusPresentation
service, and display download/status errors inline instead of failing
silently. Cover progress tracking and status presentation with tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/layaStatusPresentation.test.js

@@ -0,0 +1,43 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { presentLayaStatus } = require('../public/js/services/layaStatusPresentation.js');
+
+test('presents unknown-size downloads as indeterminate with downloaded bytes', () => {
+    const view = presentLayaStatus({
+        phase: 'downloading',
+        downloading: true,
+        progressPercent: null,
+        bytesDownloaded: 12 * 1024 * 1024,
+        totalBytes: null
+    });
+    assert.equal(view.progressIndeterminate, true);
+    assert.equal(view.progressPercent, null);
+    assert.equal(view.bytesDownloaded, 12 * 1024 * 1024);
+    assert.equal(view.showDownload, false);
+    assert.equal(view.showRemove, false);
+});
+
+test('presents known progress as a bounded percentage', () => {
+    const view = presentLayaStatus({ phase: 'downloading', downloading: true, progressPercent: 47.6 });
+    assert.equal(view.progressIndeterminate, false);
+    assert.equal(view.progressPercent, 48);
+});
+
+test('failed downloads expose retry, safe details and Standard fallback', () => {
+    const view = presentLayaStatus({
+        phase: 'failed',
+        modelPresent: false,
+        error: { message: 'Model download failed', detail: 'The optional Laya runtime is unavailable.' }
+    }, 'hybrid');
+    assert.equal(view.downloadLabel, 'Retry download');
+    assert.equal(view.showDownload, true);
+    assert.equal(view.showUseStandard, true);
+    assert.equal(view.errorMessage, 'Model download failed');
+    assert.equal(view.errorDetail, 'The optional Laya runtime is unavailable.');
+});
+
+test('ready models expose removal without download actions', () => {
+    const view = presentLayaStatus({ phase: 'ready', modelPresent: true, runtimeAvailable: true });
+    assert.equal(view.showRemove, true);
+    assert.equal(view.showDownload, false);
+});