feat(FN-2456): persist task token usage on task records

- Add schema v44 migration to persist task-level token usage totals and first/last usage timestamps on tasks
- Extend core task types, store create/update flows, and exports to round-trip token usage data
- Add migration and TaskStore regression tests for token usage persistence, null clearing, and reinitialization behavior
- Update dashboard async handling and tests to prevent post-unmount state updates and reduce flaky assertion timing

packages/core/src/__tests__/task-documents.test.ts

@@ -51,7 +51,7 @@ describe("TaskStore task documents", () => {
 
     expect(tableNames.has("task_documents")).toBe(true);
     expect(tableNames.has("task_document_revisions")).toBe(true);
-    expect(db.getSchemaVersion()).toBe(43);
+    expect(db.getSchemaVersion()).toBe(44);
 
     const index = db
       .prepare(