fix(desktop): keep macOS local services off system proxies and find user Node installs

[Luhaozhu]

Problem

On macOS, the desktop client's bundled local server could break in two independent ways:

1. System proxy intercepts loopback traffic. The local profile is a mesh of 127.0.0.1 services (backend ↔ MCP sidecars ↔ script runner ↔ internal publish callbacks) speaking httpx, which honours http_proxy/https_proxy/all_proxy. With a desktop system proxy enabled (e.g. Clash), those loopback requests were routed through the proxy, which answers 127.0.0.1 targets with 502 Bad Gateway — startup readiness failed (http://127.0.0.1:9113/mcp → 502) and site publishing errored at runtime.
2. Node detection misses per-user installs. The macOS bootstrap installer runs with the minimal GUI PATH and only probed /opt/homebrew/bin and /usr/local/bin as fallbacks. Node installed under ~/.local/bin or via nvm/fnm/mise/volta was reported missing, silently degrading React site building, Playwright screenshots, and document tools.

Additionally, the script-runner /get_file cap (10MB) was below the site-publish pack limit (40MB), so publishing a larger site (e.g. with images) failed with 413 after a successful in-sandbox tar.

Changes

• The local profile merges loopback hosts (127.0.0.1, localhost, ::1) into NO_PROXY/no_proxy at startup. Proxy variables themselves are left intact — external model/API calls may legitimately need them.
• The macOS installer probes common per-user Node locations: ~/.local/bin, Homebrew on both architectures, ~/.volta/bin, and nvm/fnm/mise version trees.
• /get_file now uses a dedicated 64MB fetch cap so publishes up to the 40MB pack limit succeed.
• Tests cover the NO_PROXY merge semantics and the installer probe list.

Testing

• tests/test_local_profile.py passes, including the new proxy-bypass test.
• cargo test --lib passes in desktop/src-tauri (installer content assertions).
• Verified end to end on macOS with a system proxy enabled: freshly packaged client auto-installs the local server, sidecar readiness passes, Node is detected from ~/.local/bin, and a site publish round-trips successfully (workspace at ~/.hugagent/workspace, hosted page served back correctly).