docs: add calendar integration spec

docs/TASKS/calendar-integrations/003-content-calendar-ui-integration.md

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+# Task: Content calendar UI integration
+
+## Goal
+
+Show imported calendar events and calendar source controls in the Content calendar.
+
+## Feature Spec
+
+- `docs/FEATURES/calendar-integrations.md`
+
+## Scope
+
+- 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 calendar source control grouped by Organization, Workspace, and My calendars.
+- 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.
+
+## Relevant Files
+
+- `frontend/src/features/content/views/ContentItemsView.vue`
+- `frontend/src/features/content/`
+- `frontend/src/plugins/api.js`
+- `frontend/src/locales/en.json`
+- `frontend/src/locales/fr.json`
+
+## Validation
+
+```bash
+cd frontend
+npm run build
+```