docs: unify project API keys and environment settings #7

Integrations now uses the same layout and controls for personal, team, and project settings. API keys and environment variables share the API Keys tab.

Update the integrations/API-key documentation and add a changelog entry covering:

• Connections, Agent Skills, MCP Servers, and API Keys share navigation, search, forms, and styling in every scope. The header identifies the current scope.
• API Keys shows saved keys and variables detected in the current project, with Add key and Import actions. Provider recommendations and category grids have been removed.
• Opening Integrations from a project's sidebar, composer, or three-dot actions menu stays in that project. The chat API-key card opens the same project editor.
• The separate code-view Environment button and RL minting menu row are removed.
• Saving project keys changes that project only; it does not copy them into workspace defaults. Personal and team settings remain available in their own Integrations pages.
• Project overrides take precedence over workspace defaults. Clearing an inherited value keeps it cleared for the project.
• Save applies keys to running project previews. Paused previews pick them up on resume, and a failed refresh is shown with a retry message.
• Existing Environment links continue to open the project API Keys tab.

Implementation: predev-app PR #1634 and PR #1639.