build: add single-command release script + portable zip artifacts

One-developer workflow: ``python build/make_release.py`` on each
target OS produces both the installer and a portable .zip for that
platform. Preflight checks PyInstaller / Pillow / iscc / hdiutil /
ditto / appimagetool and bails with install hints if anything is
missing — no half-built dist/.

New scripts:
- build/make_release.py   — orchestrator, auto-detects host OS.
- build/generate_icons.py — icon.ico / icon.icns / icon.png from
  src/gui/assets/datatools_icon_256.png (Pillow ships ICO + ICNS
  writers; no platform tooling needed).
- build/build_portable_zip.py — Win/Linux portable zip via stdlib.
- build/macos/build_zip.sh — Mac portable .app via ditto so
  bundle metadata survives.

installer.iss now adds: Quick Launch task (opt-in, legacy Win 7),
App Paths registry entry (Win+R "DataTools" works), SetupIconFile,
UninstallDisplayIcon, AppSupportURL, AppUpdatesURL.

CI workflow uploads installer + portable per platform and attaches
both to GitHub Releases on tag push.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

build/build_portable_zip.py

@@ -0,0 +1,69 @@
+"""Wrap the PyInstaller folder build into a portable .zip.
+
+Self-contained download: unzip → double-click the launcher → app runs.
+No installer, no Python install, no admin rights required.
+
+Usage:
+    python build/build_portable_zip.py <platform> <version>
+
+Where ``platform`` is one of ``win`` / ``mac`` / ``linux``. The
+script just produces a generic ``dist/DataTools/`` zip; on macOS the
+preferred portable format is the ``ditto``-wrapped .app — see
+``build/macos/build_zip.sh`` for that flow. This helper exists mainly
+for Windows + Linux, where there's no .app bundle to wrap.
+
+Output:
+    dist/DataTools-<version>-<platform>-portable.zip
+
+The zip root is the ``DataTools/`` folder so an unzip produces a
+self-contained dir the user can drop anywhere (Desktop, USB stick,
+network share). On Windows, the launcher is ``DataTools.exe`` inside
+that folder; on Linux, ``DataTools``.
+"""
+
+from __future__ import annotations
+
+import shutil
+import sys
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parent.parent
+DIST_DIR = REPO / "dist"
+BUNDLE_DIR = DIST_DIR / "DataTools"
+
+
+def main() -> int:
+    if len(sys.argv) < 3:
+        sys.stderr.write(
+            "usage: python build/build_portable_zip.py <platform> <version>\n"
+        )
+        return 2
+    platform = sys.argv[1]
+    version = sys.argv[2]
+
+    if not BUNDLE_DIR.is_dir():
+        sys.stderr.write(
+            f"Bundle dir not found at {BUNDLE_DIR}.\n"
+            "Run ``pyinstaller build/datatools.spec --clean --noconfirm`` first.\n"
+        )
+        return 1
+
+    out_stem = DIST_DIR / f"DataTools-{version}-{platform}-portable"
+    # ``make_archive`` takes a base name (no extension) and produces
+    # ``<base>.zip``. ``root_dir`` = parent of what we want compressed,
+    # ``base_dir`` = the folder name inside the archive root. This
+    # combo yields a single top-level ``DataTools/`` directory inside
+    # the .zip rather than dumping its contents loose.
+    archive = shutil.make_archive(
+        base_name=str(out_stem),
+        format="zip",
+        root_dir=str(DIST_DIR),
+        base_dir="DataTools",
+    )
+    size_mb = Path(archive).stat().st_size / (1024 * 1024)
+    print(f"wrote {archive} ({size_mb:.1f} MB)")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())