Refactor channel account management: move binding/editing to Channels, align Agents display, and simplify UX

[vcfgv]

Summary

This PR reorganizes channel-account management around the Channels page and reduces confusing/duplicated UI in both Channels and Agents.

It introduces account-scoped channel APIs, enforces clearer agent binding rules, and aligns the Agents page display with actual channel-account ownership.

What changed

Backend / API

• Added account-centric channel endpoints:
  • GET /api/channels/accounts
  • PUT /api/channels/binding
  • DELETE /api/channels/binding
  • PUT /api/channels/default-account
• Added account-scoped delete support:
  • DELETE /api/channels/config/:channelType?accountId=...
• Added aggregated channel-account view builder (config + runtime + bindings).
• Updated auto-binding compatibility behavior:
  • avoid binding random account IDs to non-existent agents (Agent not found fix)
  • keep legacy compatibility for default/main and accountId==agentId cases.
• Enforced binding constraints in agent config flow:
  • one agent cannot be bound across multiple channel types
  • within same channel type, rebinding replaces previous account binding for that agent.

Channels page UX

• Reworked configured channels into channel groups with multiple accounts.
• Support adding new accounts under the same channel type.
• Account binding is managed directly in-channel via agent selector.
• “Add account” now uses a single-step modal:
  • account ID is editable in the same config form (no multi-stage dialog).
• Removed noisy/redundant labels and status text per product feedback:
  • removed prominent binding/connection text blocks
  • removed “set as default account” button from UI
  • removed duplicated “handled by …” copy (agent selector is the source of truth).
• Fixed flicker/regression behaviors:
  • add flow no longer flashes loading before full form
  • edit flow prefetches config to reduce modal flash
  • delete uses optimistic UI removal + delayed refresh to avoid “deleted but still visible until manual refresh”.

Agents page UX

• Agents list now shows channel and account context (e.g. Feishu / Lark · Main account) instead of only channel type.
• Agent settings channel section now reflects account-level assignments from Channels.
• Removed overly noisy status emphasis in agent channel list.
• Fixed i18n fallback for default account label (default -> localized Main account).

i18n / copy cleanup

• Added and refined locale keys for zh/en/ja across channels and agents.
• Reworded several strings to better match the new ownership model and reduce ambiguity.

Docs

• Updated README.md, README.zh-CN.md, README.ja-JP.md to reflect the new channel-account/binding management flow.

Validation

• Targeted ESLint checks passed for modified backend/frontend files.
• Note: full typecheck is currently blocked by an existing unrelated workspace issue:
  • missing module use-stick-to-bottom in src/hooks/use-stick-to-bottom-instant.ts.

Related Issues

closes #445