T3: Autosample UX - fail reason + toast + pre-warm feedback

vstiAutosample: fail map thanh object {ts, reason} (cooldown check
object-aware), recordFail o API-missing/response loi/catch, export
failReasonFor. soundfontPlayer: toast ly do khi ensure null (throttle
10s) roi fallback oscillator. app.jsx: pre-warm VSTi hien thi toast khi
autosample fail.

app/static/js/services/vstiAutosample.js

@@ -19,6 +19,13 @@ window.SonicVstiAutosample = window.SonicVstiAutosample || {};
     function readFails() {
         try { return JSON.parse(localStorage.getItem(LS_FAIL_KEY) || '{}') || {}; } catch (e) { return {}; }
     }
+    function recordFail(k, reason) {
+        try {
+            var fails2 = readFails();
+            fails2[k] = { ts: Date.now(), reason: reason || 'Autosample that bai' };
+            localStorage.setItem(LS_FAIL_KEY, JSON.stringify(fails2));
+        } catch (e) {}
+    }
 
     // Key = plugin + preset → autosample lại khi đổi preset (âm preset mới).
     function keyFor(synthEngine) {
@@ -51,10 +58,13 @@ window.SonicVstiAutosample = window.SonicVstiAutosample || {};
         if (cached) return Promise.resolve(cached);
         if (_inFlight[k]) return _inFlight[k];
         var fails = readFails();
-        if (fails[k] && (Date.now() - fails[k]) < FAIL_COOLDOWN_MS) {
+        var _f = fails[k];
+        var _lastTs = (_f && typeof _f === 'object') ? _f.ts : _f;
+        if (_lastTs && (Date.now() - _lastTs) < FAIL_COOLDOWN_MS) {
             return Promise.resolve(null);
         }
         if (!window.SonicAPI || !window.SonicAPI.autosampleVsti) {
+            recordFail(k, 'API autosample khong kha dung');
             return Promise.resolve(null);
         }
         _inFlight[k] = new Promise(function (resolve) {
@@ -72,19 +82,29 @@ window.SonicVstiAutosample = window.SonicVstiAutosample || {};
                     writeMap(map);
                     resolve(res.sf_id);
                 } else {
+                    recordFail(k, (res && res.error) ? String(res.error) : 'Server khong tra sf_id');
                     resolve(null);
                 }
-            }).catch(function () {
+            }).catch(function (err) {
                 delete _inFlight[k];
-                var fails2 = readFails();
-                fails2[k] = Date.now();
-                try { localStorage.setItem(LS_FAIL_KEY, JSON.stringify(fails2)); } catch (e) {}
+                recordFail(k, (err && err.message) ? String(err.message) : 'Loi mang / server autosample');
                 resolve(null);
             });
         });
         return _inFlight[k];
     }
 
+    // Lý do autosample thất bại gần nhất (fail map) — null nếu chưa từng fail
+    // (hoặc entry cũ định dạng number). Dùng cho UI toast khi ensure() trả null.
+    function failReasonFor(synthEngine) {
+        var k = keyFor(synthEngine);
+        if (!k) return null;
+        var fails = readFails();
+        var f = fails[k];
+        if (!f || typeof f !== 'object') return null;
+        return f.reason || null;
+    }
+
     // Xoá cache + cooldown — cho phép re-sample (đổi preset/plugin hoặc debug).
     function clearCache() {
         try { localStorage.removeItem(LS_MAP_KEY); } catch (e) {}
@@ -96,6 +116,7 @@ window.SonicVstiAutosample = window.SonicVstiAutosample || {};
         sfIdFor: sfIdFor,
         entryFor: entryFor,
         ensure: ensure,
+        failReasonFor: failReasonFor,
         clearCache: clearCache
     };
 })();