fix: resolve Dependabot alerts 441 and 442

[dafuga]

Summary

• update electron-builder to 26.15.0, which resolves app-builder-lib to 26.15.0
• update electron-updater to 6.8.9, which removes the remaining older builder-util-runtime resolution
• resolve builder-util-runtime to 9.7.0

This addresses Dependabot alerts #441 and #442.

Validation

• dependency resolution: passed (app-builder-lib@26.15.0, builder-util-runtime@9.7.0)
• diff scope: passed (only package.json and yarn.lock)
• git diff --check: passed
• yarn test: failed; 29 suites passed, 31 failed, and 1 was skipped. The failing suites report that Electron did not install correctly, and 15 snapshot suites also report failures.
• yarn lint: failed; the legacy ESLint setup cannot resolve app/webpack.config.eslint.js and then exits in template-curly-spacing with Cannot read properties of null (reading 'range').
• yarn build: failed; both npm-backed build commands exit under npm 11 with Invalid property "devEngines.node".

Compatibility risk

This moves legacy Electron 12 project tooling across major versions (electron-builder 23 to 26 and electron-updater 4 to 6). The install also reports an unmet electron-builder-squirrel-windows@26.15.0 peer dependency. Packaging and updater behavior therefore need validation in the repository's supported build environment before release.