build: add Linux and Windows build scripts with NSIS installer

build-linux.sh configures and builds via CMake. build-windows.sh builds
in MSYS2/MINGW64, bundles Qt and transitive MinGW DLLs via windeployqt
plus an ldd pass, and produces an NSIS installer (installer.nsi). Ignore
the dist/ output and generated Setup.exe.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

build-linux.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+#
+# Build Ultimate Media Tracker on Linux (Qt6 + CMake).
+#
+# Usage:
+#   ./build-linux.sh           configure + build (Release)
+#   ./build-linux.sh clean     wipe the build dir first
+#   BUILD_TYPE=Debug ./build-linux.sh
+#
+set -euo pipefail
+cd "$(dirname "$0")"
+
+BUILD_DIR="build"
+BUILD_TYPE="${BUILD_TYPE:-Release}"
+
+if [[ "${1:-}" == "clean" ]]; then
+    echo "==> Removing $BUILD_DIR"
+    rm -rf "$BUILD_DIR"
+fi
+
+# Prefer Ninja for a fresh configure; reuse the existing generator otherwise
+# (passing -G on an already-configured dir errors out).
+GEN=()
+if [[ ! -f "$BUILD_DIR/CMakeCache.txt" ]] && command -v ninja >/dev/null 2>&1; then
+    GEN=(-G Ninja)
+fi
+
+echo "==> Configuring ($BUILD_TYPE)"
+cmake -S . -B "$BUILD_DIR" "${GEN[@]}" -DCMAKE_BUILD_TYPE="$BUILD_TYPE"
+
+echo "==> Building"
+cmake --build "$BUILD_DIR" -j
+
+echo
+echo "==> Done. Binary: $BUILD_DIR/umt"