perf(desktop): stop the Projects fan refetching on re-entry and running after leave (#6458)

Entering Projects fires a large fan: an exhaustive paginated relay
enumeration (projects/repos/tombstones), five 2,000-event work-item
queries plus assignment-operation scans, per-repo activity summaries,
and a local-repository filesystem scan. Measured on a large community
(101 issues / 258 PRs):

| query | measured cost |
|---|---|
| work-items (5 × 2,000-event REQs + assignment scans) | 3.5–3.9s |
| activity summaries | 4.1s |
| repository activity | 1.0–2.2s |
| local repository scan | 1.7s |

Two lifecycle bugs made the fan far more expensive than it needs to be:

- **Freshness windows guaranteed a full refetch on nearly every
re-entry** (60s enumeration, 30s work-items/activity, 10s local scan) —
i.e., the costs above were re-paid on almost every visit. Every local
write path already invalidates its keys explicitly (issue/PR mutations,
project creation, repo sync), so the short windows only served
remote-actor freshness. Raised to 5m/2m/2m with a 30m enumeration cache:
re-entries now paint from cache, and the fan re-runs at most every 2–5
minutes.
- **Leaving Projects left the whole fan running**, competing with the
next surface's channel fetches on the same relay connection. AbortSignal
is now threaded through the enumeration and assignment pagination loops
(optional params — behavior identical without a signal), and leaving the
surface cancels the work-items query. Deliberately NOT cancelled: the
enumeration (the always-mounted sidebar projects section observes it and
its 30m cache is valuable), repo snapshots and local scans (native work
that can't abort — cancelling would discard the finished result and
force the same clones again), and activity summaries (a single bounded
request).

Abort behavior is covered by red-first unit tests on both pagination
loops. Remaining follow-up (out of scope): the queries themselves want a
relay-side aggregate instead of shipping thousands of events to compute
counts client-side.

---------

Signed-off-by: Max Lampert <maxwell@squareup.com>

desktop/src/features/projects/assignmentOperationFetch.test.mjs

@@ -285,3 +285,38 @@ test("a failed assignment query surfaces as a failed section instead of silent l
 
   assert.ok(result.issues.failedSections.includes("assignments"));
 });
+
+test("fetchAssignmentOperationEvents stops paginating once its signal aborts", async () => {
+  // A permanently-full page would paginate forever without the cursor; abort
+  // after the first page and require the loop to stop with AbortError.
+  const controller = new AbortController();
+  let fetches = 0;
+  const fullPage = (until) =>
+    Array.from({ length: 500 }, (_, index) => ({
+      id: `${until ?? "head"}-${index}`.padEnd(64, "0"),
+      kind: 1,
+      pubkey: "a".repeat(64),
+      created_at: 1_000_000 - fetches * 1_000 - index,
+      content: JSON.stringify({ type: "assign" }),
+      tags: [],
+    }));
+  const fetchEvents = async (filter) => {
+    fetches += 1;
+    // Bound the fake: without the abort support the loop would paginate
+    // forever (each page has a fresh cursor) and OOM the test run.
+    if (fetches > 3) throw new Error("kept paginating after abort");
+    const page = fullPage(filter.until);
+    controller.abort();
+    return page;
+  };
+
+  await assert.rejects(
+    fetchAssignmentOperationEvents(
+      ["issue".padEnd(64, "1")],
+      fetchEvents,
+      controller.signal,
+    ),
+    (error) => error.name === "AbortError",
+  );
+  assert.equal(fetches, 1);
+});