feat: add HTTP/HTTPS proxy support via undici ProxyAgent

[RyanLee-Dev]

Summary

• Use undici ProxyAgent + setGlobalDispatcher at startup — all fetch() calls go through proxy automatically, zero call-site changes
• Support config file proxy: mmx config set --key proxy --value http://HOST:PORT
• Proxy priority: HTTPS_PROXY > HTTP_PROXY > ALL_PROXY > config file
• Improve network error hints to guide users toward proxy setup instead of misleading "check proxy settings"

Supersedes #101, closes #96

Changes

File	What
main.ts	Inject ProxyAgent at startup
config/schema.ts	Add proxy field to ConfigFile
commands/config/set.ts	Allow mmx config set --key proxy with URL validation
commands/config/show.ts	Display configured proxy
errors/handler.ts	Honest network error hints
build.ts	Mark undici as external
package.json	Add undici dependency

Why not PR #101's approach

PR #101 adds 120-line custom proxy module + replaces every fetch() call site (12 files, +655/-32). This PR achieves the same with setGlobalDispatcher — 8 files, +45/-7, zero invasiveness.

Test plan

• HTTPS_PROXY=http://127.0.0.1:7890 mmx quota show — traffic goes through proxy (verified via local CONNECT proxy)
• mmx config set --key proxy --value http://127.0.0.1:7890 then mmx quota show — same
• No proxy configured — direct connection, no warnings
• TypeScript typecheck passes
• 143 tests pass