Render join requests inline in inbox like approvals and other work items

Join requests were displayed in a separate card-style section below the main
inbox list. This moves them into the unified work items feed so they sort
chronologically alongside issues, approvals, and failed runs—matching the
inline treatment hiring requests already receive.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/lib/inbox.test.ts

@@ -296,6 +296,7 @@ describe("inbox helpers", () => {
       }).map((item) => {
         if (item.kind === "issue") return `issue:${item.issue.id}`;
         if (item.kind === "approval") return `approval:${item.approval.id}`;
+        if (item.kind === "join_request") return `join:${item.joinRequest.id}`;
         return `run:${item.run.id}`;
       }),
     ).toEqual([
@@ -305,6 +306,37 @@ describe("inbox helpers", () => {
     ]);
   });
 
+  it("mixes join requests into the inbox feed by most recent activity", () => {
+    const issue = makeIssue("1", true);
+    issue.lastExternalCommentAt = new Date("2026-03-11T04:00:00.000Z");
+
+    const joinRequest = makeJoinRequest("join-1");
+    joinRequest.createdAt = new Date("2026-03-11T03:00:00.000Z");
+
+    const approval = makeApprovalWithTimestamps(
+      "approval-oldest",
+      "pending",
+      "2026-03-11T02:00:00.000Z",
+    );
+
+    expect(
+      getInboxWorkItems({
+        issues: [issue],
+        approvals: [approval],
+        joinRequests: [joinRequest],
+      }).map((item) => {
+        if (item.kind === "issue") return `issue:${item.issue.id}`;
+        if (item.kind === "approval") return `approval:${item.approval.id}`;
+        if (item.kind === "join_request") return `join:${item.joinRequest.id}`;
+        return `run:${item.run.id}`;
+      }),
+    ).toEqual([
+      "issue:1",
+      "join:join-1",
+      "approval:approval-oldest",
+    ]);
+  });
+
   it("can include sections on recent without forcing them to be unread", () => {
     expect(
       shouldShowInboxSection({

ui/src/lib/inbox.ts

@@ -28,6 +28,11 @@ export type InboxWorkItem =
       kind: "failed_run";
       timestamp: number;
       run: HeartbeatRun;
+    }
+  | {
+      kind: "join_request";
+      timestamp: number;
+      joinRequest: JoinRequest;
     };
 
 export interface InboxBadgeData {
@@ -152,10 +157,12 @@ export function getInboxWorkItems({
   issues,
   approvals,
   failedRuns = [],
+  joinRequests = [],
 }: {
   issues: Issue[];
   approvals: Approval[];
   failedRuns?: HeartbeatRun[];
+  joinRequests?: JoinRequest[];
 }): InboxWorkItem[] {
   return [
     ...issues.map((issue) => ({
@@ -173,6 +180,11 @@ export function getInboxWorkItems({
       timestamp: normalizeTimestamp(run.createdAt),
       run,
     })),
+    ...joinRequests.map((joinRequest) => ({
+      kind: "join_request" as const,
+      timestamp: normalizeTimestamp(joinRequest.createdAt),
+      joinRequest,
+    })),
   ].sort((a, b) => {
     const timestampDiff = b.timestamp - a.timestamp;
     if (timestampDiff !== 0) return timestampDiff;