feat: add configurable retry cooldown and polish config UX

[mxyhi]

Summary

• make retryable failure cooldown configurable and reset runtime cooldown state on config reload
• expose the cooldown setting in the config form and document the updated retry/cooldown policy
• improve the upstreams table with tighter columns, truncation handling, and hover tooltips
• auto-save config changes after debounce and avoid retrying the same failed draft forever

Commit Order

1. feat(core): make retry cooldown configurable
2. feat(config): expose retry cooldown setting
3. refactor(config): improve upstreams table readability
4. refactor(config): auto-save config changes

Testing

• cargo test -p token_proxy_core
• pnpm run i18n:compile
• pnpm exec tsc --noEmit
• pnpm vitest run src/features/config/form.test.ts src/features/config/AppView.test.tsx src/features/config/ConfigScreen.test.tsx src/features/config/cards/upstreams/constants.test.ts src/features/config/cards/upstreams/table.test.tsx

[mxyhi]

已补充跟进修复，主要覆盖本轮评审关注点：

• 保存配置后，运行中的代理会按需自动 reload / restart，停止中的代理保持停止
• 为 retryable_failure_cooldown_secs 增加构建期溢出校验，并在 selector 侧用 checked_add 兜底避免 panic
• 配置页移除常驻保存按钮，仅在自动保存失败时显示局部“重试保存”入口
• 同步更新中英文文案与 README，语义对齐为“自动保存 + 自动应用运行时配置”

已复跑验证：

• pnpm run i18n:compile
• pnpm exec tsc --noEmit
• pnpm vitest run src/features/config/form.test.ts src/features/config/AppView.test.tsx src/features/config/ConfigScreen.test.tsx src/features/config/cards/upstreams/constants.test.ts src/features/config/cards/upstreams/table.test.tsx
• cargo test -p token_proxy_core
• cargo check --manifest-path src-tauri/Cargo.toml