feat(proxy): add global desktop proxy settings

[devRickyyy]

Summary

• add a persisted desktop proxy preference with system and local-port modes
• apply the selected proxy to daemon HTTP traffic, Electron sessions, and newly spawned agents, terminals, Git commands, managed runtimes, and workspace apps
• expose the setting in the General preferences UI with port validation and localized English/Chinese copy
• update SQLite migrations, OpenAPI/event contracts, generated clients, tests, and the durable network proxy convention

Users can keep following the operating-system proxy or route Tutti through an HTTP proxy at 127.0.0.1:<port>. Saving applies immediately to in-process traffic and future child processes; existing child processes must be restarted to inherit new environment variables.

Verification

• pnpm check:changed --tail-lines 80 — passed 16 lanes after rebasing onto the latest origin/main
• pre-push pnpm check:full — passed 18 tasks
• desktop tests — 1493 passed, 2 skipped
• Go full tests and build
• TypeScript typecheck, desktop production build, Go/TypeScript lint
• OpenAPI/event generated checks and i18n validation

Checklist

• I kept the change focused on one concern.
• I updated documentation when behavior, setup, or contributor workflow changed.
• I updated all README/CONTRIBUTING language variants when changing their English source.
• I ran the lowest meaningful local checks for the changed surface.
• My commits are signed off with DCO when required: git commit -s.