feat(04-03): add cross-platform build script and build:all npm script

- Create scripts/build-all-platforms.mjs that detects current platform, invokes electron-builder via build-desktop.mjs, verifies artifacts in build/release/, and reports remaining platforms
- Add "build:all" script entry to package.json for D-07 cross-platform build validation

scripts/build-all-platforms.mjs

@@ -0,0 +1,118 @@
+#!/usr/bin/env node
+/**
+ * build-all-platforms.mjs
+ * Builds Electron app artifacts for the current platform and reports
+ * which platforms still need native builds on their respective hosts.
+ *
+ * Usage: node scripts/build-all-platforms.mjs [--skip-vite]
+ *
+ * Per D-07: All three platforms (macOS, Windows, Linux) must produce
+ * valid build artifacts.
+ *
+ * electron-builder does NOT support true cross-compilation for all targets:
+ *   - macOS DMG/zip requires macOS
+ *   - Windows NSIS/portable requires Windows (or Wine)
+ *   - Linux AppImage requires Linux (or Docker)
+ *
+ * This script builds for the CURRENT platform, verifies artifacts, and
+ * documents remaining platforms that must be built on their native hosts.
+ */
+import { execFileSync } from "node:child_process";
+import { existsSync } from "node:fs";
+import { readdir } from "node:fs/promises";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const rootDir = join(__dirname, "..");
+const releaseDir = join(rootDir, "build", "release");
+const skipVite = process.argv.includes("--skip-vite");
+
+const PLATFORM_FLAGS = {
+  darwin: "--mac",
+  win32: "--win",
+  linux: "--linux",
+};
+
+const PLATFORM_NAMES = {
+  darwin: "macOS",
+  win32: "Windows",
+  linux: "Linux",
+};
+
+const EXPECTED_EXTENSIONS = {
+  darwin: [".dmg", ".zip"],
+  win32: [".exe"],
+  linux: [".AppImage"],
+};
+
+const currentPlatform = process.platform;
+const platformFlag = PLATFORM_FLAGS[currentPlatform];
+
+if (!platformFlag) {
+  console.error(`[build-all] Unsupported platform: ${currentPlatform}`);
+  process.exit(1);
+}
+
+console.log(`[build-all] Building for ${PLATFORM_NAMES[currentPlatform]} (${currentPlatform})...`);
+
+// Step 1: Run electron-vite build (unless --skip-vite)
+if (!skipVite) {
+  console.log("[build-all] Running electron-vite build...");
+  execFileSync("pnpm", ["electron-vite", "build"], {
+    stdio: "inherit",
+    cwd: rootDir,
+    shell: currentPlatform === "win32",
+  });
+}
+
+// Step 2: Run build-desktop.mjs with the current platform flag
+console.log(`[build-all] Running electron-builder ${platformFlag}...`);
+execFileSync("node", [join(__dirname, "build-desktop.mjs"), platformFlag], {
+  stdio: "inherit",
+  cwd: rootDir,
+  env: {
+    ...process.env,
+    CSC_IDENTITY_AUTO_DISCOVERY: process.env.CSC_IDENTITY_AUTO_DISCOVERY ?? "false",
+  },
+  shell: currentPlatform === "win32",
+});
+
+// Step 3: Verify build artifacts exist
+if (!existsSync(releaseDir)) {
+  console.error(`[build-all] ERROR: Release directory not found: ${releaseDir}`);
+  process.exit(1);
+}
+
+const files = await readdir(releaseDir);
+const expectedExts = EXPECTED_EXTENSIONS[currentPlatform];
+const foundArtifacts = files.filter((f) =>
+  expectedExts.some((ext) => f.endsWith(ext)),
+);
+
+if (foundArtifacts.length === 0) {
+  console.error(
+    `[build-all] ERROR: No ${PLATFORM_NAMES[currentPlatform]} artifacts found in ${releaseDir}. ` +
+    `Expected files with extensions: ${expectedExts.join(", ")}`,
+  );
+  process.exit(1);
+}
+
+console.log(`\n[build-all] SUCCESS: ${PLATFORM_NAMES[currentPlatform]} build artifacts:`);
+for (const artifact of foundArtifacts) {
+  console.log(`  - ${artifact}`);
+}
+
+// Step 4: Report remaining platforms
+const otherPlatforms = Object.entries(PLATFORM_NAMES)
+  .filter(([key]) => key !== currentPlatform)
+  .map(([, name]) => name);
+
+if (otherPlatforms.length > 0) {
+  console.log(`\n[build-all] NOTE: Build on these platforms for full D-07 coverage:`);
+  for (const name of otherPlatforms) {
+    console.log(`  - ${name}: run 'pnpm build:all' on a ${name} host`);
+  }
+}
+
+console.log("\n[build-all] Done.");