feat(mcp): add Zed client to MCP setup sheet

[datlechin]

Summary

• Add Zed to the in-app MCP Setup sheet (Settings > Integrations) and the MCP Token Reveal sheet, alongside Claude Desktop, Claude Code, and Cursor.
• Snippets target Zed's "Add Custom Server" dialog: paste the inner server entry only (Zed wraps it under context_servers itself), and include args: [] because the dialog's Local-tab schema rejects the field even though Zed's documented schema treats it as optional.
• Docs section now shows both paths: the Agent Panel UI ("+ Add Custom Server" → Local) and direct ~/.config/zed/settings.json editing under context_servers.

Test plan

• Open Settings > Integrations > MCP Setup, pick Zed, copy the snippet, paste into Zed's Agent Panel → + Add Custom Server → Local tab. Verify validator passes and tablepro shows a green dot.
• Mint a token in Settings > Integrations > Authentication, switch the reveal sheet to Zed, paste into Zed's + Add Custom Server → Remote tab. Verify the connection works.
• Open docs/external-api/mcp-clients.mdx Zed section in the rendered docs site and confirm both code blocks render correctly.

Notes for reviewer

• The xcstrings file was NOT updated in this PR, so the 4 new String(localized:) calls (3 step lines + the "Zed" display label) will fall back to the source-language English at runtime until the next localization sync pass.
• Remote-tab snippet shape is inferred from the Local-tab pattern; only Local was visually verified against the validator.

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