feat(ui): add Telegram bot connection UI to Sharing page (#296)

[vybe]

Summary

• Add TelegramChannelPanel.vue component to the Agent Detail Sharing page for connecting/managing Telegram bots per agent
• Mirrors the existing Slack channel panel pattern with bot token input, status display, verify, and disconnect flows
• Uses api.js auth client (Invariant security: Fix token logging and add HTML reports to gitignore #7), password input for token handling, webhook status warning banner

Changes

• New: src/frontend/src/components/TelegramChannelPanel.vue — Self-contained panel component
• Modified: src/frontend/src/components/SharingPanel.vue — Import and render Telegram panel

Key Decisions

• Bot token input uses type="password" and ref is cleared immediately on success (security)
• Shows webhook warning banner when bot is connected but webhook_url is null (common self-hosted issue)
• "Verify" button calls getMe (labeled accurately — does not send a message without chat_id)
• Requires explicit disconnect before reconnecting (prevents silent webhook_secret regeneration)
• Active chats display deferred — no backend endpoint exists for telegram_chat_links yet

Test Plan

• Navigate to Agent Detail → Sharing tab → Telegram Bot section visible
• Paste a BotFather token → "Connect Bot" → shows @username and t.me link
• Click "Verify" → shows "Bot @username is operational" success message
• Click "Disconnect" → returns to token input state
• Paste a token already bound to another agent → shows 409 error with agent name
• Connect bot without public URL configured → yellow webhook warning banner appears
• Token input is type="password" (masked)
• Dark mode renders correctly

Closes #296

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