refactor: Integrate PreferencesMiddleware and update handlers

Complete middleware integration with comprehensive testing:

1. Middleware Integration
   - Added PreferencesMiddleware to middleware chain in routes
   - Order: Recovery → Logger → SecurityHeaders → Preferences → Mux
   - Reads all preference cookies once per request
   - Stores in context for handlers to access

2. Handler Updates
   - cv_pages.go: Home handler uses middleware.GetPreferences()
   - cv_htmx.go: All toggle handlers use middleware preferences
   - Eliminated manual cookie reading in handlers
   - Migration logic handled entirely by middleware

3. Comprehensive Middleware Tests
   - Created preferences_test.go with 10+ test functions
   - Tests: default values, migrations, cookie setting, context access
   - Verified: extended→long, true→show, false→hide migrations
   - All tests passing

Benefits:
- Performance: Cookies read once per request (not multiple times)
- Consistency: All handlers get same preference values
- Maintainability: Migration logic centralized in middleware
- Testability: Easy to mock preferences via context

Testing:
- All unit tests pass (handlers + middleware)
- Build succeeds
- No breaking changes

internal/routes/routes.go

@@ -41,9 +41,12 @@ func Setup(cvHandler *handlers.CVHandler, healthHandler *handlers.HealthHandler)
 	mux.Handle("/static/", middleware.CacheControl(staticHandler))
 
 	// Apply comprehensive middleware chain
+	// Order: Recovery → Logger → SecurityHeaders → Preferences → Mux
 	handler := middleware.Recovery(
 		middleware.Logger(
-			middleware.SecurityHeaders(mux),
+			middleware.SecurityHeaders(
+				middleware.PreferencesMiddleware(mux),
+			),
 		),
 	)