Customised Analytics for admins and users (#4687)

Adds granular privacy controls for analytics - splits single
enableAnalytics toggle into separate PostHog and Scarf controls with
improved admin
  UX.

  Backend Changes

  Configuration (ApplicationProperties.java)
  - Added enablePosthog and enableScarf boolean fields
- New methods: isPosthogEnabled(), isScarfEnabled() (null = enabled when
analytics is on)

  Services
- PostHogService: Now checks isPosthogEnabled() instead of
isAnalyticsEnabled()
  - ConfigController: Exposes new flags via API
- SettingsController: Changed endpoint from @RequestBody to
@RequestParam

  Frontend Changes

  Architecture
- Converted useAppConfig hook → AppConfigContext provider for global
access
  - Added refetch() method for config updates without reload

  New Features
1. AdminAnalyticsChoiceModal: First-launch modal when enableAnalytics
=== null
    - Enable/disable without editing YAML
    - Includes documentation link
  2. Scarf Tracking System: Modular utility with React hook wrapper
    - Respects config + per-service cookie consent
    - Works from any code location (React or vanilla JS)
3. Enhanced Cookie Consent: Per-service toggles (PostHog and Scarf
separate)

  Integration
  - App.tsx: Added AppConfigProvider + scarf initializer
  - HomePage.tsx: Shows admin modal when needed
  - index.tsx: PostHog opt-out by default, service-level consent

  Key Benefits

✅ Backward compatible (null defaults to enabled)
✅ Granular control per analytics service
✅ First-launch admin modal (no YAML editing)
✅ Privacy-focused with opt-out defaults
✅ API-based config updates

---------

Co-authored-by: Connor Yoh <connor@stirlingpdf.com>

frontend/public/locales/en-GB/translation.json

@@ -250,6 +250,7 @@
     "title": "Do you want make Stirling PDF better?",
     "paragraph1": "Stirling PDF has opt in analytics to help us improve the product. We do not track any personal information or file contents.",
     "paragraph2": "Please consider enabling analytics to help Stirling-PDF grow and to allow us to understand our users better.",
+    "learnMore": "Learn more",
     "enable": "Enable analytics",
     "disable": "Disable analytics",
     "settings": "You can change the settings for analytics in the config/settings.yml file"
@@ -3371,6 +3372,10 @@
         "title": "Analytics",
         "description": "These cookies help us understand how our tools are being used, so we can focus on building the features our community values most. Rest assured—Stirling PDF cannot and will never track the content of the documents you work with."
       }
+    },
+    "services": {
+      "posthog": "PostHog Analytics",
+      "scarf": "Scarf Pixel"
     }
   },
   "removeMetadata": {