feat(ui): Add integrations page driven by the integrations API

[nfebe]

Task

Show the user which integrations their backend has installed and their status, driven by the backend integrations endpoint.

Actions

• Fetch GET /api/v1/integrations. Each item has: key, name, description, category, icon, feature_key, configured.
• Render an Integrations page/section listing each integration with its name, category, icon, and a state badge (configured vs not).
• Group or label by category (e.g. bank sync, document analytics).
• Empty state when the list is empty (an open install with no integrations installed returns []).

Acceptance

• The page reflects exactly what GET /api/v1/integrations returns; an empty response shows the empty state.
• Each item shows name, category, and whether it is configured.
• Auth follows the app's normal handling (the endpoint requires authentication).

Out of scope

• Per-integration connect/disconnect actions and the plan selection / checkout flow. Those are separate and depend on the cloud billing work.