Feature: Add elegant PWA loading screen

Implemented a professional loading screen for the PWA to eliminate the
"hang and wait" experience during app initialization.

Changes:
- Added full-screen gradient loading overlay with TeleShop branding
- Implemented animated triple-ring spinner with smooth animations
- Added automatic removal after PWA initialization (500ms fade-out)
- Included 5-second fallback timeout to prevent infinite loading
- Updated service worker cache version to v2
- Enhanced JWT validation to detect test/temporary keys
- Updated appsettings.json with secure JWT key

Design Features:
- Purple/blue gradient background matching brand colors
- Pulsing logo animation for visual interest
- Staggered spinner rings with cubic-bezier easing
- Fade-in-out loading text animation
- Mobile-responsive design (scales appropriately on all devices)

Technical Implementation:
- Loading screen visible by default (no FOUC)
- Removed via JavaScript when PWA manager initialization completes
- Graceful fade-out animation before DOM removal
- Console logging for debugging

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

LittleShop/appsettings.json

@@ -3,7 +3,7 @@
     "DefaultConnection": "Data Source=littleshop.db"
   },
   "Jwt": {
-    "Key": "ThisIsATemporaryKeyFor-TestingPurposesOnlyGenerateSecureKey1234567890ABCDEF",
+    "Key": "9xKmN3pQwR7vYzH4bFtJ8sLcE2nW6aVgDhU5kXmP1oZiAqBjCrTy0MxSfGdIlPeWuO",
     "Issuer": "LittleShop",
     "Audience": "LittleShop",
     "ExpiryInHours": 24