fix(vst-gui): borderless fx-gui window + anchored re-show (mouse stable)

- RenderFxJob: offscreen fx-gui window now WS_POPUP (borderless) so
  GetWindowRect == client rect; anchored placement over the embed panel is
  exact. WS_OVERLAPPEDWINDOW put the client ~31px lower/right -> cursor
  jumped on hover and clicks hit the webview (relay rejected).
- AdjustWindowRect sizes to the window's own style (no caption for POPUP).
- FxGuiServer: mousemove only SetCursorPos while dragging (buttons!=0);
  hover no longer teleports the cursor.
- FxGuiServer /show: skip SetWindowPos/resize when rect unchanged (frontend
  now re-POSTs /show every ~500ms to re-anchor on scroll/resize).
- app.jsx VstGuiEmbed: drop shownRef one-shot; re-POST /show each refresh.
- Rebuilt bridge (6 locations) + Tauri release (embedded sidecar) + v=202608190002.

native_bridge/src/RenderFxJob.cpp

@@ -667,7 +667,9 @@ public:
             int w = rect.right - rect.left;
             int h = rect.bottom - rect.top;
             RECT r = { 0, 0, w, h };
-            AdjustWindowRect(&r, WS_OVERLAPPEDWINDOW, FALSE);
+            // Size to the window's own style: borderless (WS_POPUP) needs no
+            // caption/border adjustment.
+            AdjustWindowRect(&r, GetWindowLongPtrA(hwnd, GWL_STYLE), FALSE);
             SetWindowPos(hwnd, nullptr, 0, 0, r.right - r.left, r.bottom - r.top,
                          SWP_NOMOVE | SWP_NOZORDER | SWP_NOACTIVATE);
             view->onSize(&rect);  // some Skia/OpenGL editors skip first paint
@@ -1192,7 +1194,11 @@ static int fxGuiCreateWindow(const std::string& name, bool offscreen,
     const int x = offscreen ? -20000 : CW_USEDEFAULT;
     const int y = offscreen ? -20000 : CW_USEDEFAULT;
     const DWORD exStyle = offscreen ? WS_EX_TOOLWINDOW : 0;
-    const DWORD style = (offscreen ? (WS_OVERLAPPEDWINDOW & ~WS_MINIMIZEBOX) : WS_OVERLAPPEDWINDOW)
+    // Borderless (WS_POPUP): GetWindowRect == client rect, so anchored
+    // placement over the embed panel is exact. A captioned/bordered window
+    // (WS_OVERLAPPEDWINDOW) put the client area ~31px lower/right of the
+    // requested rect -> cursor jumped on hover and clicks landed on the webview.
+    const DWORD style = (offscreen ? WS_POPUP : WS_OVERLAPPEDWINDOW)
                         | WS_VISIBLE | WS_CLIPCHILDREN | WS_CLIPSIBLINGS;
     HWND hwnd = CreateWindowExA(exStyle, kFxGuiWndClass,
                                 (name.empty() ? "VST FX" : name).c_str(),