Task: Content calendar UI integration

Goal

Show imported calendar events and calendar source controls in the Content calendar.

Add calendar source data loading to the Content feature.
Render imported events as read-only calendar context entries in Month, Week, and Upcoming views.
Style imported events distinctly from Socialize content items and apply source colors.
Add a compact calendar source dropdown next to the Content calendar view selector.
Add visibility toggles for displayed sources.
Add Add calendar as the last source-control entry.
Add an add-calendar flow that supports curated catalog search and custom .ics URL subscriptions according to user permissions.
Keep imported events lower visual priority than actual content work on crowded days.
Add a quick action to create content from an imported event, prefilled with date and event title.

cd frontend
npm run build