fix: roundtrip markdown & SortOptionTest count

Fix toMarkdown() missing newline between title header and content/checklist,
which caused fromMarkdown() to merge title + first content line on roundtrip.

Fix SortOptionTest: update expected count from 5 to 7 to reflect
CREATION_DATE and CREATION_DATE_DESC added in v1.11.0.

Author: inventory69

android/app/src/main/java/dev/dettmer/simplenotes/models/Note.kt

@@ -111,13 +111,13 @@ type: ${noteType.name.lowercase()}$sortLine
         """.trimIndent()
 
         return when (noteType) {
-            NoteType.TEXT -> header + content
+            NoteType.TEXT -> header + "\n" + content
             NoteType.CHECKLIST -> {
                 val checklistMarkdown = checklistItems?.sortedBy { it.order }?.joinToString("\n") { item ->
                     val checkbox = if (item.isChecked) "[x]" else "[ ]"
                     "- $checkbox ${item.text}"
                 }.orEmpty()
-                header + checklistMarkdown
+                header + "\n" + checklistMarkdown
             }
         }
     }

android/app/src/test/java/dev/dettmer/simplenotes/models/SortOptionTest.kt

@@ -77,8 +77,8 @@ class SortOptionTest {
     // ═══════════════════════════════════════════════
 
     @Test
-    fun `ChecklistSortOption has all 5 values`() {
-        assertEquals(5, ChecklistSortOption.entries.size)
+    fun `ChecklistSortOption has all 7 values`() {
+        assertEquals(7, ChecklistSortOption.entries.size)
     }
 
     @Test