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

native_bridge/src/NativeInstrumentEngine.cpp

@@ -47,8 +47,7 @@ bool FluidSynthInstrument::init(double sampleRate, uint32_t maxBlockSize) {
 
 void FluidSynthInstrument::selectProgram(uint32_t channel, uint32_t bank, uint32_t program) {
     if (!synth) return;
-    fluid_synth_bank_select(FS_SYNTH, channel, bank);
-    fluid_synth_program_change(FS_SYNTH, channel, program);
+    fluid_synth_program_select(FS_SYNTH, channel, sfontId, bank, program);
 }
 
 void FluidSynthInstrument::noteOn(uint32_t channel, uint32_t pitch, float velocity, uint32_t sampleOffset) {
@@ -69,7 +68,7 @@ void FluidSynthInstrument::controlChange(uint32_t channel, uint32_t cc, uint32_t
 
 void FluidSynthInstrument::programChange(uint32_t channel, uint32_t program) {
     if (!synth) return;
-    fluid_synth_program_change(FS_SYNTH, channel, program);
+    fluid_synth_program_select(FS_SYNTH, channel, sfontId, 0, program);
 }
 
 void FluidSynthInstrument::pitchBend(uint32_t channel, uint32_t bend14) {