fix: inject HTTP proxy env vars into node subprocess calls

[likehopper]

Problem

On Nextcloud installations behind an HTTP proxy, the InstallDeps
repair step fails with ETIMEDOUT errors when downloading
libtensorflow and ffmpeg binaries.

Node subprocesses spawned via exec() do not inherit proxy
environment variables, even when proxy is configured in
Nextcloud's config.php.

Solution

• Inject IConfig dependency to read the system proxy setting
• Add getProxyEnv() helper method that builds the proxy env prefix
• Prepend HTTPS_PROXY and HTTP_PROXY to the exec() commands
  in runTfjsInstall(), runTfjsGpuInstall() and runFfmpegInstall()

Testing

Verified on Nextcloud 33.0.4 behind a Squid proxy (OPNsense)
where installation previously failed with ETIMEDOUT on
storage.googleapis.com.

[marcelklehr]

Nicely done, thank you

[marcelklehr]

Could you please run composer run cs:fix once and commit the result to make sure the code adheres to our coding standards?

[github-actions]

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[marcelklehr]

Thank you!!