Provider update check: respect the configured npm registry

[oliynykmax]

The provider update check (fetchNpmLatestVersion, apps/server/src/provider/providerMaintenance.ts) always requests https://registry.npmjs.org/<pkg>/latest. On networks where the public registry is blocked and npm goes through an internal mirror (set via .npmrc / npm_config_registry), the request is blocked, times out after 4s, and the advisory is always unknown. On networks that monitor egress, it also creates blocked-connection noise every hour for each provider.

Turning off provider update checks stops the request, but then you lose the update notice entirely.

Proposal: use npm_config_registry (the environment variable npm itself reads) when it's set, and fall back to registry.npmjs.org. Reading .npmrc would cover more setups but is more code; the env var alone is a very small change.

Happy to send a small PR if this is wanted.

Version: 0.0.43-nightly.20260925.2237, macOS, Claude Code and OpenCode providers.