Drop ErrorView's widget prop in favour of the host bridge

ErrorView needed a widget only to decide whether to offer a close button
or a link home. That prop was threaded down through ErrorPage, RichError
and GroupCallErrorBoundary from seven call sites, to answer the single
question of whether the host can dismiss Element Call — which the host
bridge now answers directly through the presence of close().

Read the bridge from context in ErrorView and remove the prop, along with
the plumbing that carried it.

No functional change: the rendered output is unchanged, as the existing
snapshot confirms.

Move GroupCallView onto the host bridge

GroupCallView asked the widget API to keep it on screen, to close it, and
to tell it when a preloaded call should join. Route all three through the
host bridge and drop the widget prop.

src/HostBridge.test.ts

@@ -0,0 +1,101 @@
+/*
+Copyright 2026 Element Creations Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { describe, expect, test, vi } from "vitest";
+import { type WidgetApi } from "matrix-widget-api";
+import EventEmitter from "events";
+
+import { createWidgetHostBridge, nullHostBridge } from "./HostBridge";
+import { type WidgetHelpers } from "./widget";
+
+function mockWidget(api: Partial<WidgetApi>): WidgetHelpers {
+  return {
+    api: api as WidgetApi,
+    lazyActions: new EventEmitter(),
+    client: Promise.resolve(),
+  } as unknown as WidgetHelpers;
+}
+
+describe("createWidgetHostBridge", () => {
+  describe("downloadMedia", () => {
+    const mxcUri = "mxc://example.org/alice-avatar";
+
+    test("passes a Blob through unchanged", async () => {
+      const file = new Blob([]);
+      const bridge = createWidgetHostBridge(
+        mockWidget({ downloadFile: vi.fn().mockResolvedValue({ file }) }),
+      );
+
+      await expect(bridge.downloadMedia!(mxcUri)).resolves.toBe(file);
+    });
+
+    test("decodes a base64 string into a Blob", async () => {
+      const bridge = createWidgetHostBridge(
+        mockWidget({
+          // "hello" in base64
+          downloadFile: vi.fn().mockResolvedValue({ file: "aGVsbG8=" }),
+        }),
+      );
+
+      const blob = await bridge.downloadMedia!(mxcUri);
+
+      expect(blob).toBeInstanceOf(Blob);
+      // The five decoded bytes, rather than the eight characters of base64 —
+      // which is what we'd get if the string were stored verbatim.
+      expect(blob.size).toBe(5);
+    });
+
+    test("rejects a file format it does not understand", async () => {
+      const bridge = createWidgetHostBridge(
+        mockWidget({ downloadFile: vi.fn().mockResolvedValue({ file: 42 }) }),
+      );
+
+      await expect(bridge.downloadMedia!(mxcUri)).rejects.toThrow(
+        "Downloaded file format is not supported",
+      );
+    });
+  });
+
+  describe("supportsReactions", () => {
+    const capabilities = [
+      "org.matrix.msc2762.send.event:m.reaction",
+      "org.matrix.msc2762.send.event:m.room.redaction",
+      "org.matrix.msc2762.receive.event:m.reaction",
+      "org.matrix.msc2762.receive.event:m.room.redaction",
+    ];
+
+    test("is true when the host grants every reaction capability", () => {
+      const bridge = createWidgetHostBridge(
+        mockWidget({ hasCapability: () => true }),
+      );
+
+      expect(bridge.supportsReactions).toBe(true);
+    });
+
+    test.each(capabilities)("is false without %s", (missing) => {
+      const bridge = createWidgetHostBridge(
+        mockWidget({ hasCapability: (c) => c !== missing }),
+      );
+
+      expect(bridge.supportsReactions).toBe(false);
+    });
+  });
+});
+
+describe("nullHostBridge", () => {
+  test("offers no way to close, so the interface falls back to navigation", () => {
+    expect(nullHostBridge.close).toBeUndefined();
+  });
+
+  test("offers no media download, so Element Call uses its own client", () => {
+    expect(nullHostBridge.downloadMedia).toBeUndefined();
+  });
+
+  test("supports reactions, since nothing is mediating its homeserver access", () => {
+    expect(nullHostBridge.supportsReactions).toBe(true);
+  });
+});