feat: add global outbound proxy settings

[SaladDay]

Summary

• add persistent global outbound proxy configuration through CLI and TUI settings
• follow the upstream model: store one proxy URL, apply it to the shared HTTP client, and fall back to standard proxy environment variables when unset
• support HTTP, HTTPS, SOCKS5, and SOCKS5H URLs with optional username/password authentication
• give saved configuration precedence over environment variables and existing provider-specific proxy fallback
• apply changes immediately in the current process and reload existing Unix daemon/foreground proxy workers through the existing runtime orchestration
• keep credentials visible on explicit configuration surfaces while redacting logs, errors, daemon status, crash diagnostics, and Debug output

User interface

• CLI: settings outbound-proxy show/set/clear; set "" also clears the saved value
• TUI: Settings → Global Outbound Proxy; Enter edits, validates, and immediately commits the focused field
• fields can be entered in any order, including credentials before the URL
• invalid URLs remain available for correction and produce readable localized errors
• clearing the URL returns to environment-variable fallback
• TUI confirms when environment proxy variables are present; CLI prints a warning and proceeds without a --yes flag

Scope

This does not add proxy connectivity testing, proxy scanning, per-provider proxy settings, README documentation, keyring integration, or unrelated HTTP-client hardening.

Validation

• full sandboxed library suite: 4,131 passed, 0 failed, 2 ignored
• focused outbound-proxy CLI/TUI/daemon tests: 12 passed
• global proxy service tests: 6 passed
• HTTP client proxy tests: 9 passed
• future-schema compatible proxy-setting read test: 1 passed
• sandboxed CLI set/show/blank-clear smoke test passed, including password-only authentication
• cargo build --bin cc-switch
• cargo check --bin cc-switch
• cargo fmt --check
• git diff --check
• final independent blind review reported no actionable findings

Closes #419