Add custom interval days support for task frequency

- Add customIntervalDays field to Kotlin models (TaskResponse, TaskCreateRequest, TaskUpdateRequest)
- Update Android AddTaskDialog to show interval field only for "Custom" frequency
- Update Android EditTaskScreen for custom frequency support
- Update iOS TaskFormView for custom frequency support
- Fix preview data in TaskCard and TasksSection to include new field
- Add customIntervalDays to OnboardingFirstTaskView

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

composeApp/src/jvmMain/kotlin/com/example/casera/storage/TaskCacheManager.jvm.kt

@@ -1,10 +1,10 @@
-package com.casera.storage
+package com.example.casera.storage
 
-import java.io.File
 import java.util.prefs.Preferences
 
 /**
  * JVM implementation of TaskCacheManager using Java Preferences.
+ * Note: JVM/Desktop doesn't have widgets, so dirty flag methods are no-ops.
  */
 actual class TaskCacheManager {
     private val prefs = Preferences.userRoot().node(NODE_NAME)
@@ -23,9 +23,33 @@ actual class TaskCacheManager {
         prefs.flush()
     }
 
+    /**
+     * Check if tasks need refresh. JVM doesn't have widgets, always returns false.
+     */
+    actual fun areTasksDirty(): Boolean {
+        return prefs.getBoolean(KEY_DIRTY_FLAG, false)
+    }
+
+    /**
+     * Mark tasks as dirty. JVM doesn't have widgets but supports the interface.
+     */
+    actual fun markTasksDirty() {
+        prefs.putBoolean(KEY_DIRTY_FLAG, true)
+        prefs.flush()
+    }
+
+    /**
+     * Clear the dirty flag.
+     */
+    actual fun clearDirtyFlag() {
+        prefs.putBoolean(KEY_DIRTY_FLAG, false)
+        prefs.flush()
+    }
+
     companion object {
         private const val NODE_NAME = "com.casera.cache"
         private const val KEY_TASKS = "cached_tasks"
+        private const val KEY_DIRTY_FLAG = "tasks_dirty"
 
         @Volatile
         private var instance: TaskCacheManager? = null