fix: improve Feishu/Lark channel modal UX and refine default main agent naming

[vcfgv]

This PR delivers three focused fixes on Channels/Agents UX:

1. Prevent accidental modal close during text selection in Feishu/Lark config

• In channel config modal, backdrop close now triggers on backdrop mousedown only (target check), and modal card stops mousedown propagation.
• This prevents the modal from closing when users drag-select text (e.g. Account ID) and release outside the modal.

2. Add “Bound Agent:” label before account binding dropdown on configured channel cards

• Improves clarity of the account-to-agent binding action in each configured account row.
• Added localized copy in en/zh/ja channel locale files.

3. Rename default main agent alias from Main to Main Agent

• Updated backend default alias constant so implicit/default main agent displays as Main Agent consistently.