fix: use CALDAV_ENABLED for calendar feature flag (#1070)

* fix: add FF_ENABLE_CALDAV feature flag for CalDAV functionality

Introduces a new dedicated feature flag for CalDAV sync that checks
both FF_ENABLE_CALDAV and CALDAV_ENABLED environment variables. This
allows the CalDAV tab to appear in profile settings when users set
CALDAV_ENABLED=true as documented.

The existing FF_ENABLE_CALENDAR remains unchanged as it's for a
separate (hidden) calendar feature.

Changes:
- Added 'caldav' feature flag to backend service (checks FF_ENABLE_CALDAV
  or CALDAV_ENABLED)
- Updated frontend FeatureFlags interface to include 'caldav'
- Changed CalDAV tab to use 'caldav' feature flag instead of 'calendar'
- Added FF_ENABLE_CALDAV to .env.example, .env.test, Dockerfile, and CI

Fixes #1048

* fix: add caldav property to all FeatureFlags initializations

Fixes TypeScript errors where FeatureFlags objects were missing the
new caldav property in:
- frontend/utils/featureFlags.ts (defaultFlags and error return)
- frontend/components/Navbar.tsx
- frontend/components/Sidebar.tsx
- frontend/components/Sidebar/SidebarNav.tsx

backend/.env.example

@@ -29,6 +29,7 @@ DISABLE_TELEGRAM=false
 
 # Feature Flags
 FF_ENABLE_BACKUPS=false
+FF_ENABLE_CALDAV=false
 FF_ENABLE_CALENDAR=false
 FF_ENABLE_HABITS=false
 FF_ENABLE_MCP=false

backend/.env.test

@@ -4,5 +4,6 @@ TUDUDI_SESSION_SECRET=test-secret-key-for-testing
 TUDUDI_ALLOWED_ORIGINS=http://localhost:3000,http://localhost:8080
 
 FF_ENABLE_BACKUPS=false
+FF_ENABLE_CALDAV=false
 FF_ENABLE_CALENDAR=false
 FF_ENABLE_HABITS=false

backend/modules/feature-flags/service.js

@@ -8,6 +8,9 @@ class FeatureFlagsService {
         return {
             backups: process.env.FF_ENABLE_BACKUPS === 'true',
             calendar: process.env.FF_ENABLE_CALENDAR === 'true',
+            caldav:
+                process.env.FF_ENABLE_CALDAV === 'true' ||
+                process.env.CALDAV_ENABLED === 'true',
             habits: process.env.FF_ENABLE_HABITS === 'true',
             mcp: process.env.FF_ENABLE_MCP === 'true',
         };