feat(web): make MCP Servers editable in agent settings

[field-claw]

Summary

Makes the agent settings page's MCP Servers section editable (it was a read-only JSON dump). Pairs with the core MCP adapter in PR #216 — this UI lets users configure the servers that adapter connects at agent run time.

What changed

• Replaced the read-only

   with a per-server editor: name input + config JSON textarea, add/remove rows.

• Live JSON parse validation per row (inline error if the config isn't valid JSON).
• submit diffs the edited list against the loaded config and only sends mcpServers when changed (PUT config's partial-update model).
• config stays free-form JSON so users paste the exact stdio/sse shape the core adapter consumes — no UI-side schema assumptions.
• i18n (en + zh) updated; removed the "(read-only)" label.

Notes

• Web typecheck shows pre-existing sibling-package subpath resolution noise (@prismshadow/penguin-core/omnimessage etc.) that resolves under a full monorepo build; �gent-settings-page.tsx itself is clean.
• Not browser-verified in this environment (no display); logic is straightforward controlled inputs.

Generated with Claude as a PenguinHarness MCP evaluation follow-up.