docs(dashboard): use existing test helpers and document automatic cleanup

Fixed integration tests to use existing helpers from test_helpers.go:

Changes:
- Replace getUserUUIDFromToken() with getTestUserID(t, db) helper ✅
- Replace parseUUID() with uuid.MustParse() ✅
- Add explicit comments about automatic cleanup via t.Cleanup() ✅

Test Helpers Used (all from test_helpers.go):
- setupTestServer(t) - creates test server with automatic cleanup
- loginTestUser(t, ts, db) - logs in admin user
- loginRegularUser(t, ts, db) - logs in regular user
- setupDeviceTest(t) - creates server + user + device + library
- getTestUserID(t, db) - gets/creates admin test user UUID
- uuid.MustParse() - parses UUID strings

Cleanup Pattern:
- Automatic via t.Cleanup() inside setupTestServer()
- Registered automatically when setupTestServer() is called
- No manual defer setup.Close() needed
- Runs even if test fails or panics
- Cleanup order: queue → connections → server → database

Dashboard-Specific Helper:
- updateDashboardPreferences() - only for dashboard testing
- Saves dashboard preferences for test scenarios

Benefits:
- Uses proven, existing helpers (no reinventing the wheel)
- Automatic cleanup prevents resource leaks
- Follows project testing patterns exactly
- Less custom code = fewer bugs

CAROUSEL_DASHBOARD_PLAN.md

@@ -2192,6 +2192,9 @@ import (
 
 func TestDashboardAPI_GetSections(t *testing.T) {
     setup := setupTestServer(t)
+    // Note: t.Cleanup() is automatically registered inside setupTestServer()
+    // No manual cleanup needed - Close() called automatically when test completes
+    
     adminToken := loginTestUser(t, setup.Server, setup.DB)
 
     // Create test library with media items
@@ -2309,6 +2312,8 @@ func TestDashboardAPI_GetSections(t *testing.T) {
 
 func TestDashboardAPI_UserPreferences(t *testing.T) {
     setup := setupTestServer(t)
+    // Automatic cleanup via t.Cleanup() - no manual cleanup needed
+    
     adminToken := loginTestUser(t, setup.Server, setup.DB)
 
     // Create test library
@@ -2316,9 +2321,9 @@ func TestDashboardAPI_UserPreferences(t *testing.T) {
     libraryID := deviceSetup.CreateLibrary(t, "Test Library", "ebooks")
 
     t.Run("GetSections_WithHiddenSections", func(t *testing.T) {
-        // First, save preferences to hide a section
-        userUUID := getUserUUIDFromToken(t, setup.DB, adminToken)
-        libUUID := parseUUID(t, libraryID)
+        // Get admin user UUID using existing helper
+        userUUID := getTestUserID(t, setup.DB)
+        libUUID := uuid.MustParse(libraryID)
 
         // Save dashboard preferences with hidden sections
         updateDashboardPreferences(t, setup.DB, userUUID, libUUID, map[string]interface{}{
@@ -2349,8 +2354,8 @@ func TestDashboardAPI_UserPreferences(t *testing.T) {
     })
 
     t.Run("GetSections_WithCustomOrder", func(t *testing.T) {
-        userUUID := getUserUUIDFromToken(t, setup.DB, adminToken)
-        libUUID := parseUUID(t, libraryID)
+        userUUID := getTestUserID(t, setup.DB)
+        libUUID := uuid.MustParse(libraryID)
 
         // Save dashboard preferences with custom order
         customOrder := []string{"recently-read", "continue-reading", "in-progress"}
@@ -2390,6 +2395,8 @@ func TestDashboardAPI_UserPreferences(t *testing.T) {
 
 func TestDashboardSSR_Page(t *testing.T) {
     setup := setupTestServer(t)
+    // Automatic cleanup via t.Cleanup() - no manual cleanup needed
+    
     adminToken := loginTestUser(t, setup.Server, setup.DB)
 
     // Create test library
@@ -2431,6 +2438,7 @@ func TestDashboardSSR_Page(t *testing.T) {
 // Helper functions for dashboard tests
 
 // updateDashboardPreferences saves dashboard preferences for testing
+// NOTE: This is specific to dashboard testing - not in test_helpers.go
 func updateDashboardPreferences(t *testing.T, db *database.Queries, userID, libraryID uuid.UUID, prefs map[string]interface{}) {
     hiddenSections := prefs["hidden_sections"].([]string)
     sectionOrder := prefs["section_order"].([]string)
@@ -2446,9 +2454,7 @@ func updateDashboardPreferences(t *testing.T, db *database.Queries, userID, libr
 }
 ```
 
-**Key Helper Functions Available:**
-
-From `test_helpers.go`:
+**Key Helper Functions Available (from test_helpers.go):**
 
 ```go
 // setupTestServer creates complete test environment with auto cleanup
@@ -2471,6 +2477,20 @@ adminUUID := getTestUserID(t, setup.DB)
 
 // getRegularUserID - gets/creates regular test user UUID
 userUUID := getRegularUserID(t, setup.DB)
+
+// uuid.MustParse - parse UUID string (from uuid package)
+libUUID := uuid.MustParse(libraryID)
+```
+
+**Dashboard-Specific Helper (created for these tests):**
+
+```go
+// updateDashboardPreferences - saves dashboard preferences for testing
+// NOTE: Only needed for dashboard testing, not a general helper
+updateDashboardPreferences(t, setup.DB, userUUID, libUUID, map[string]interface{}{
+    "hidden_sections": []string{"recently-added"},
+    "section_order":   []string{"recently-read", "continue-reading"},
+})
 ```
 
 **Cleanup Pattern:**
@@ -2619,7 +2639,13 @@ on('click', '[data-action="open-dashboard-settings"]', () => {
 
 ### Integration Tests (Phase 15):
 - [ ] Integration tests created (`cmd/server/tests/dashboard_test.go`)
-- [ ] Uses setupTestServer() helper
+- [ ] Uses `setupTestServer(t)` helper with automatic `t.Cleanup()` ✅
+- [ ] Uses `loginTestUser(t, ts, db)` helper ✅
+- [ ] Uses `loginRegularUser(t, ts, db)` helper ✅
+- [ ] Uses `setupDeviceTest(t)` helper ✅
+- [ ] Uses `getTestUserID(t, db)` helper ✅
+- [ ] Uses `uuid.MustParse()` for UUID parsing ✅
+- [ ] Automatic cleanup (no manual defer/Close needed) ✅
 - [ ] Three-context testing (no auth, user, admin)
 - [ ] GET /api/dashboard/sections tested
 - [ ] User preferences tested (hidden sections, custom order)