feat: integrations and MCP settings as full page instead of side overlay

[YUVARAJ-R-ai]

User Story

As a user who wants to manage integrations and MCP connections, I want them to open as a dedicated full page (like Claude's Customise page) so that I have more space to configure services without a cramped overlay.

What to Build

Create a /settings/integrations route (a Next.js page) that renders the full integrations + MCP connection UI currently shown in the SettingsModal.tsx side panel. The sidebar should have a direct link to this page. The page should follow the same layout as the Claude Customise page — sidebar navigation on the left with sections (Skills, Connectors), main content area on the right.

Acceptance Criteria

• /settings/integrations renders a full-page integrations view
• All existing MCP connect/disconnect flows work identically on the new page
• Sidebar navigation links to the new page
• Page layout matches the reference design (left nav + main content)
• No regression in MCP auth flows (PAT and OAuth)