feat(playback): add per-user transcoding controls (#375)

* feat(playback): add per-user transcoding controls

* fix(playback): enforce forced video transcode permission

* chore: address transcode control review feedback

* fix(playback): recheck transcode permission on audio switch

---------

Co-authored-by: Quick104 <31828688+Quick104@users.noreply.github.com>

web/src/player/playback-errors.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, it } from "vitest";
+
+import { describeTranscodingPolicyError } from "./playback-errors";
+import { PlayerFetchError } from "./player-fetch";
+
+describe("describeTranscodingPolicyError", () => {
+  it("describes disabled video transcoding", () => {
+    const error = new PlayerFetchError(
+      403,
+      "Transcoding is disabled for your user",
+      "transcoding_disabled",
+    );
+
+    expect(describeTranscodingPolicyError(error)).toEqual({
+      title: "Transcoding is disabled",
+      message: "Transcoding is disabled for your user. Ask your server administrator for access.",
+    });
+  });
+
+  it("describes disabled audio transcoding", () => {
+    const error = new PlayerFetchError(
+      403,
+      "Audio transcoding is disabled for your user",
+      "audio_transcoding_disabled",
+    );
+
+    expect(describeTranscodingPolicyError(error)).toEqual({
+      title: "Audio transcoding is disabled",
+      message:
+        "This item requires audio conversion, but audio transcoding is disabled for your user.",
+    });
+  });
+
+  it("ignores unrelated player errors", () => {
+    expect(
+      describeTranscodingPolicyError(new PlayerFetchError(500, "Failed", "internal_error")),
+    ).toBeNull();
+  });
+});