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/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);