fix(activity): guard insight generation against stale handles

A generation that settled after the range changed (or after a newer
generation started) ran its completion callbacks unconditionally: it
cleared the current handle -- leaving the new generation unabortable --
and could overwrite the panel with a result for the previous range.

Tag each generation with a version, bumped on start and on abort, and
skip the handle, insight, error, and phase updates when the settling
generation is no longer current.

Author: wesm

frontend/src/lib/components/activity/ActivityInsight.svelte

@@ -27,6 +27,10 @@
 
   // Guards stale fetch responses when the range changes mid-flight.
   let fetchVersion = 0;
+  // Bumped on every generation start and on abort, so a generation that
+  // settles after the range changed (or after a newer one started) is
+  // ignored instead of clobbering the current handle or panel state.
+  let genVersion = 0;
   // The in-flight generation, so we can abort it on range change/unmount.
   let handle: GenerateInsightHandle | null = null;
 
@@ -62,6 +66,8 @@
   function abortGeneration() {
     handle?.abort();
     handle = null;
+    // Invalidate the aborted generation so its late settle is a no-op.
+    genVersion++;
   }
 
   $effect(() => {
@@ -109,7 +115,8 @@
     phase = "starting";
     error = null;
 
-    handle = generateInsight(
+    const v = ++genVersion;
+    const current = generateInsight(
       {
         type: "daily_activity",
         date_from: dateFrom,
@@ -118,17 +125,21 @@
         agent: "claude",
       },
       (p) => {
+        if (v !== genVersion) return;
         phase = p;
       },
     );
+    handle = current;
 
-    handle.done
+    current.done
       .then((result) => {
+        if (v !== genVersion) return;
         handle = null;
         insight = result;
         generating = false;
       })
       .catch((e) => {
+        if (v !== genVersion) return;
         handle = null;
         if (e instanceof DOMException && e.name === "AbortError") {
           return;

frontend/src/lib/components/activity/ActivityInsight.test.ts

@@ -83,6 +83,40 @@ describe("ActivityInsight", () => {
     );
   });
 
+  it("ignores a generation that settles after the range changed", async () => {
+    let resolveStale!: (insight: unknown) => void;
+    const abortStale = vi.fn();
+    mocks.generateInsight.mockReturnValueOnce({
+      abort: abortStale,
+      done: new Promise((r) => {
+        resolveStale = r;
+      }),
+    });
+    const { rerender } = render(ActivityInsight, {
+      dateFrom: "2026-06-15", dateTo: "2026-06-21",
+    });
+    await settle();
+
+    // Start a generation for the first range.
+    await fireEvent.click(screen.getByRole("button", { name: /generate/i }));
+    expect(mocks.generateInsight).toHaveBeenCalledTimes(1);
+
+    // Range change aborts and invalidates the in-flight generation.
+    await rerender({ dateFrom: "2026-06-08", dateTo: "2026-06-14" });
+    await settle();
+    expect(abortStale).toHaveBeenCalled();
+
+    // The aborted generation settles late; its result must not reach the panel.
+    resolveStale({
+      id: 99, project: null, content: "STALE RESULT", type: "daily_activity",
+      date_from: "2026-06-15", date_to: "2026-06-21", agent: "claude",
+      model: null, prompt: null, created_at: "2026-06-21T00:00:00Z",
+    });
+    await settle();
+
+    expect(document.body.textContent).not.toContain("STALE RESULT");
+  });
+
   it("prefills the Insights page range and navigates", async () => {
     render(ActivityInsight, { dateFrom: "2026-06-15", dateTo: "2026-06-21" });
     await settle();