feat: Add HTTP/HTTPS proxy support for Telegram API connections

[Copilot]

Telegram API requests had no proxy support, making the integration non-functional in regions where Telegram is blocked.

Changes

Core proxy utility (telegram-utils.ts)

• getTelegramProxyUrl() reads telegram_proxy_url from SQLite settings
• proxyFetchOptions() validates the URL (http/https only), instantiates undici.ProxyAgent (bundled with Node.js 22+ — zero new dependencies), and returns { dispatcher } to spread into fetch() init

// Before
const res = await fetch(url, { method: 'POST', body: JSON.stringify(params) });

// After
const res = await fetch(url, {
  method: 'POST',
  body: JSON.stringify(params),
  ...proxyFetchOptions(),
} as RequestInit);

Coverage — proxy applied to all 5 outbound Telegram fetch sites:

• callTelegramApi() (used by all bot methods + sendMessageDraft)
• resolveBotIdentity() (getMe) in telegram-adapter.ts
• Long-polling getUpdates loop in telegram-adapter.ts
• getFile API call in telegram-media.ts
• File binary download in telegram-media.ts

Settings persistence (route.ts)

• telegram_proxy_url added to TELEGRAM_KEYS for read/write via the existing settings API

UI (TelegramBridgeSection.tsx)

• "Proxy Server" input field added to the Bot Credentials card
• Client-side validation rejects non-http:///https:// URLs before saving

i18n — 5 new keys added to en.ts and zh.ts (label, description, placeholder, hint, two validation error messages)

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