ci: add GitHub Action to build Windows EXE using wada.webp icon

[tans]

Motivation

• Provide an automated way to produce a Windows portable EXE for the app from CI.
• Use the existing wada.webp as the application icon during packaging.
• Make releases easier by producing build artifacts on tag or manual dispatch.

Description

• Added workflow .github/workflows/build-windows-exe.yml that runs on workflow_dispatch and push tags v* and performs checkout, npm ci, icon conversion, build and artifact upload.
• The workflow converts wada.webp to build/icon.ico using ImageMagick via magick convert before packaging.
• Build step uses npx electron-builder --win portable to create a Windows portable EXE and uploads dist/*.exe as an artifact.
• Updated package.json to add scripts.build:win, a devDependencies.electron-builder entry, and a build configuration (appId, productName, directories.output=dist, buildResources=build, files, and win.target=portable with icon=build/icon.ico).

Testing

• Ran syntax checks with node --check main.js, node --check preload.js, and node --check renderer/app.js, which completed successfully.
• Verified npm ci and npx electron-builder --win portable are the commands used by the workflow (no full CI run in this environment).

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Codex Task

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[chatgpt-codex-connector]

Replace npm ci with a lockfile-compatible install

The workflow’s install step will fail on every run because npm ci only works when package-lock.json/npm-shrinkwrap.json is present, but this repo currently only tracks bun.lock and this commit does not add an npm lockfile. In that state, the job exits at dependency installation (EUSAGE) before packaging, so tag builds cannot produce the Windows EXE artifact.

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