feat/mcp-edit-stdio-sources

[tsrines]

• Stdio MCP sources can now be edited from the UI — previously the edit screen only displayed the saved command and told users to delete and recreate the source.
• The edit form supports updating the command, arguments, environment variables, and a new working directory field, and re-discovers the server's tools on save.
• A failed save (e.g. a typo'd command) leaves the existing source intact instead of corrupting it, and an unchanged save is a no-op so the underlying process isn't restarted needlessly.
• The Add and Edit screens now share one component for stdio fields, so the two forms stay visually and behaviorally in sync.