feat: bundle tkinter into Windows portable build

Download _tkinter.pyd, tkinter package, and Tcl/Tk DLLs from the
official Python nuget package and copy them into the embedded Python
directory. This enables the screen overlay visualization during
calibration in the portable build.

build-dist-windows.sh

@@ -85,6 +85,74 @@ if ! grep -q '\.\./app/src' "$PTH_FILE"; then
 fi
 echo "  Patched $(basename "$PTH_FILE")"
 
+# ── Bundle tkinter into embedded Python ───────────────────────
+# Embedded Python doesn't include tkinter. We download it from the
+# official Windows Python nuget package (same version) which contains
+# the _tkinter.pyd, tkinter/ package, and Tcl/Tk DLLs.
+
+echo "[3b/8] Bundling tkinter for screen overlay support..."
+
+# Python minor version for nuget package (e.g., 3.11.9 -> 3.11)
+PYTHON_MINOR="${PYTHON_VERSION%.*}"
+
+# Download the full Python nuget package (contains all stdlib + DLLs)
+NUGET_URL="https://www.nuget.org/api/v2/package/python/${PYTHON_VERSION}"
+NUGET_PKG="$BUILD_DIR/python-nuget.zip"
+if [ ! -f "$NUGET_PKG" ]; then
+    curl -sL "$NUGET_URL" -o "$NUGET_PKG"
+fi
+
+NUGET_DIR="$BUILD_DIR/python-nuget"
+rm -rf "$NUGET_DIR"
+mkdir -p "$NUGET_DIR"
+unzip -qo "$NUGET_PKG" -d "$NUGET_DIR"
+
+# Copy _tkinter.pyd (the C extension)
+TKINTER_PYD=$(find "$NUGET_DIR" -name "_tkinter.pyd" | head -1)
+if [ -n "$TKINTER_PYD" ]; then
+    cp "$TKINTER_PYD" "$PYTHON_DIR/"
+    echo "  Copied _tkinter.pyd"
+else
+    echo "  WARNING: _tkinter.pyd not found in nuget package"
+fi
+
+# Copy tkinter Python package from the stdlib zip or Lib/
+# The nuget package has Lib/tkinter/
+TKINTER_PKG=$(find "$NUGET_DIR" -type d -name "tkinter" | head -1)
+if [ -n "$TKINTER_PKG" ]; then
+    mkdir -p "$PYTHON_DIR/Lib"
+    cp -r "$TKINTER_PKG" "$PYTHON_DIR/Lib/tkinter"
+    echo "  Copied tkinter/ package"
+else
+    echo "  WARNING: tkinter package not found in nuget package"
+fi
+
+# Copy Tcl/Tk DLLs (tcl86t.dll, tk86t.dll, etc.)
+for dll in tcl86t.dll tk86t.dll; do
+    DLL_PATH=$(find "$NUGET_DIR" -name "$dll" | head -1)
+    if [ -n "$DLL_PATH" ]; then
+        cp "$DLL_PATH" "$PYTHON_DIR/"
+        echo "  Copied $dll"
+    fi
+done
+
+# Copy Tcl/Tk data directories (tcl8.6, tk8.6)
+for tcldir in tcl8.6 tk8.6; do
+    TCL_PATH=$(find "$NUGET_DIR" -type d -name "$tcldir" | head -1)
+    if [ -n "$TCL_PATH" ]; then
+        cp -r "$TCL_PATH" "$PYTHON_DIR/$tcldir"
+        echo "  Copied $tcldir/"
+    fi
+done
+
+# Add Lib to ._pth so tkinter package is importable
+if ! grep -q '^Lib$' "$PTH_FILE"; then
+    echo 'Lib' >> "$PTH_FILE"
+fi
+
+rm -rf "$NUGET_DIR"
+echo "  tkinter bundled successfully"
+
 # ── Download pip and install into embedded Python ────────────
 
 echo "[4/8] Installing pip into embedded Python..."