feat(mastering): remove redundant VST FX MASTERBUS row from panel

VSTs are now added via the chain '+' button (addVstToChain mirrors into
vstFxChain for offline render), so the old bottom row (slot list + '+
Thêm VST FX...' select + offline preview) duplicated the chain and is
removed.

app/static/js/app.jsx

@@ -13189,30 +13189,6 @@ const addMasterVst = (path) => { const fx = (masterFxList || []).find(f => f.pat
           </button>
         </div>
 
-        {/* VST FX MASTERBUS (native_bridge --render-fx — áp dụng khi Export/Offline render) */}
-        <div className="bg-slate-950 border-b border-teal-900/60 px-4 py-2 flex flex-col gap-2 shrink-0">
-          <div className="flex items-center justify-between flex-wrap gap-2">
-            <div className="flex items-center gap-2 min-w-0">
-              <span className="text-[10px] font-bold text-teal-400 uppercase tracking-widest oz-font-mono shrink-0">VST FX MASTERBUS:</span>
-              <span className="text-[9px] text-slate-500 oz-font-mono truncate">Plugin VST3 (Ozone...) — realtime browser vẫn WebAudio builtin; VST áp dụng lúc Export/Offline render (preview = âm export)</span>
-            </div>
-            <div className="flex items-center gap-2 shrink-0">
-              <button onClick={previewMasterFx} className="px-2 py-1 bg-teal-800 hover:bg-teal-700 text-teal-100 text-[10px] font-semibold rounded transition flex items-center gap-1">
-                <i data-lucide="play" className="w-3 h-3"></i> Nghe thử (render offline)
-              </button>
-              <label className="flex items-center gap-1 text-[10px] text-slate-400 oz-font-mono cursor-pointer">
-                <input type="checkbox" checked={!!ozState.vstBypass} onChange={e => setOzState(prev => ({ ...prev, vstBypass: e.target.checked }))} className="w-3 h-3" /> Bypass
-              </label>
-            </div>
-          </div>
-          {masterVstChain.length === 0 && <div className="text-[10px] text-slate-500 oz-font-mono">Chưa có VST FX masterbus — chọn plugin bên dưới (Ozone...) để chạy chain sau khi mix WebAudio.</div>}
-          {masterVstChain.map((s, idx) => vstRow(s, idx, toggleMasterVst, changeMasterVstPlugin, uploadMasterVstPreset, moveMasterVst, removeMasterVst, masterFxList, masterVstChain.length, openMasterVstGui))}
-          <select value={addMasterVstSel} onChange={e => { setAddMasterVstSel(e.target.value); if (e.target.value) { addMasterVst(e.target.value); setAddMasterVstSel(''); } }} className="w-full bg-slate-950 border border-dashed border-slate-700 hover:border-teal-600 text-teal-300 rounded px-2 py-1.5 text-[11px] outline-none" title="Thêm VST FX">
-            <option value="">+ Thêm VST FX...</option>
-            {(masterFxList || []).map(f => <option key={f.path} value={f.path}>{f.name || f.id}</option>)}
-          </select>
-        </div>
-
         {/* MAIN WORKSPACE */}
         <main className="flex-1 flex overflow-hidden min-h-0">