Implement Phase 6: undo/redo with command pattern

- UndoableAction interface with AddStrokeAction and DeleteStrokeAction
- UndoManager: undo/redo stacks (depth 50), canUndo/canRedo StateFlows
- EditorViewModel: stroke operations routed through UndoManager,
  visual callbacks sync canvas view without full DB reload
- Toolbar: undo/redo buttons with enabled state
- 9 unit tests for UndoManager

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/main/kotlin/net/metacircular/engpad/undo/StrokeActions.kt

@@ -0,0 +1,43 @@
+package net.metacircular.engpad.undo
+
+import net.metacircular.engpad.data.model.Stroke
+import net.metacircular.engpad.data.repository.PageRepository
+
+class AddStrokeAction(
+    private val stroke: Stroke,
+    private val repository: PageRepository,
+    private val onExecute: (Stroke) -> Unit,
+    private val onUndo: (Long) -> Unit,
+) : UndoableAction {
+    override val description = "Add stroke"
+    private var insertedId: Long = 0
+
+    override suspend fun execute() {
+        insertedId = repository.addStroke(stroke)
+        onExecute(stroke.copy(id = insertedId))
+    }
+
+    override suspend fun undo() {
+        repository.deleteStroke(insertedId)
+        onUndo(insertedId)
+    }
+}
+
+class DeleteStrokeAction(
+    private val stroke: Stroke,
+    private val repository: PageRepository,
+    private val onExecute: (Long) -> Unit,
+    private val onUndo: (Stroke) -> Unit,
+) : UndoableAction {
+    override val description = "Delete stroke"
+
+    override suspend fun execute() {
+        repository.deleteStroke(stroke.id)
+        onExecute(stroke.id)
+    }
+
+    override suspend fun undo() {
+        val id = repository.addStroke(stroke)
+        onUndo(stroke.copy(id = id))
+    }
+}