test(notifications): the center re-says its rows after a switch, a repeat coalesces by content; e2e helper fails with a cause

The center's rows (title, message, refusal, live button) were never rendered across a language
switch, and nothing held the structural compare that coalescing now needs. The channel-link helper
swallowed whatever ended its poll into a bare false: a link that goes away mid-look now counts as
not hit, and no hittable link fails with the poll's own message.

frontend/src/lib/notifications.coalesce.dom.test.ts

@@ -0,0 +1,37 @@
+// @vitest-environment jsdom
+//
+// A burst of the same notice is one entry with a count (×N), not a pile of copies. The texts are
+// objects now (what to say, built afresh by every msg() call), so "the same" is the same key and
+// values, never the same object: a compare by reference would let every repeat stack.
+import { afterEach, describe, expect, it } from "vitest";
+import { clearAll, getNotifications, msg, notify, raw } from "./notifications";
+
+afterEach(() => clearAll());
+
+describe("coalescing a repeated notice", () => {
+  it("folds the same key and values into one entry, counted", () => {
+    notify({
+      level: "error",
+      title: msg("errors.title"),
+      message: msg("errors.reason.unreachable"),
+    });
+    notify({
+      level: "error",
+      title: msg("errors.title"),
+      message: msg("errors.reason.unreachable"),
+    });
+    expect(getNotifications().map((n) => n.repeat)).toEqual([2]);
+  });
+
+  it("folds the same server refusal too", () => {
+    notify({ level: "error", message: { detail: { code: "not_found", reason: "view" } } });
+    notify({ level: "error", message: { detail: { code: "not_found", reason: "view" } } });
+    expect(getNotifications().map((n) => n.repeat)).toEqual([2]);
+  });
+
+  it("keeps two notices apart when only a value differs", () => {
+    notify({ message: msg("channels.notify.hidden", { name: raw("One") }) });
+    notify({ message: msg("channels.notify.hidden", { name: raw("Two") }) });
+    expect(getNotifications().map((n) => n.repeat)).toEqual([1, 1]);
+  });
+});