Fix AI proxy origin in Tauri, SF2 program_select, VSTi GUI reopen + sample rate bridge

- aiGateway.js: use window.API_BASE_URL (engine port) instead of window.location.origin (tauri://localhost asset protocol returned index.html -> JSON parse error); check content-type before parsing response
- NativeInstrumentEngine.cpp: fluid_synth_program_select for SF2 instrument + program change
- Vst3Instrument.cpp: activate all audio/event buses, channel=0 forced, GUI resize/reopen, kEvent using fix
- main.cpp: VST GUI reopen via WM_DESTROY, OleInitialize, steady_clock loop
- lib.rs: BridgeSampleRate state, restart_bridge_with_sample_rate command, borrow fix
- app.jsx/nativeBridgeService.js: isMidiTrack widening, bridge masterbus connect, held MIDI notes, sample-rate restart, openNativeGUI channel

app/static/js/services/aiGateway.js

@@ -294,7 +294,7 @@ ${rules.join('\n')}` },
   async function callLLM({ provider, model, apiKey, baseUrl, messages, tools, toolChoice }) {
     const base = baseUrl.replace(/\/$/, '');
     const url = `${base}/chat/completions`;
-    const origin = window.location.origin;
+    const origin = (window.API_BASE_URL || window.location.origin).replace(/\/+$/, '');
     const urlOrigin = parseOrigin(url);
     const appOrigin = parseOrigin(origin);
     const sameOrigin = urlOrigin === appOrigin;
@@ -341,7 +341,21 @@ ${rules.join('\n')}` },
       throw new Error(detail);
     }
 
-    return await response.json();
+    const ct = (response.headers.get('content-type') || '').toLowerCase();
+    if (!ct.includes('application/json') && !ct.includes('text/json')) {
+      const raw = (await response.text()).slice(0, 300);
+      throw new Error(`Phản hồi không phải JSON (HTTP ${response.status}, ${ct || 'no content-type'}).
+Có thể proxy trỏ sai địa chỉ hoặc server trả trang HTML.
+Raw: ${raw}`);
+    }
+
+    try {
+      return await response.json();
+    } catch (e) {
+      const raw = (await response.text()).slice(0, 300);
+      throw new Error(`JSON parse lỗi từ server AI (HTTP ${response.status}): ${e.message}
+Raw: ${raw}`);
+    }
   }
 
   function extractFunctionCalls(completion) {

app/static/js/services/nativeBridgeService.js

@@ -135,10 +135,13 @@
     /**
      * 3. OPEN FLOATING CHILD WINDOW NATIVE GUI
      */
-    openNativeGUI: async function (pluginId) {
+    openNativeGUI: async function (pluginId, channel) {
       if (!this._tauri()) return false;
       try {
-        await window.__TAURI__.core.invoke('open_vst_gui', { pluginId: pluginId });
+        await window.__TAURI__.core.invoke('open_vst_gui', {
+          pluginId: pluginId,
+          channel: channel === undefined ? 0 : channel
+        });
         return true;
       } catch (e) {
         console.warn('[BridgeService] open_vst_gui:', e);