feat: PipedreamPicker UI component in ConnectorPicker

[VictorGjn]

Context

Pipedream backend (client + routes + store) created in #133 but no frontend component yet.

Changes needed

1. src/components/PipedreamPicker.tsx — Search Pipedream apps, connect accounts, show connected
2. Integrate into ConnectorPicker.tsx as a tab alongside native connectors
3. Settings page: Pipedream configuration (projectId, clientId, clientSecret)
4. Connected accounts list with disconnect button

Depends on: #133 merge + mount routes (#144)