fix(audio): expose sample rate setting in Convert Audio (#561)

The Convert Audio tool promised configurable bitrate, sample rate, and channel count, but only format and bitrate were exposed. Adds an optional sampleRate setting (8000 to 96000 Hz, omitted = preserve source) wired through the Zod schema, the FFmpeg -ar flag, the standalone settings panel, and the pipeline builder controls.

Impossible combinations fail loudly instead of degrading silently: MP3 + 96000 Hz is rejected (libmp3lame caps at 48 kHz), and MP3 bitrates above the encoder ceiling at low rates (64 kbps at 8 kHz, 160 kbps at 16/22.05 kHz) are rejected rather than clamped. The UI offers only legal combinations and sanitizes stored pipeline settings on load.

Docs updated in English plus all 20 localized pages with refreshed i18n_source_hash stamps; two new UI strings added to all 21 locales.

Fixes #558

tests/unit/web/pipeline-controls.test.tsx

@@ -1,6 +1,6 @@
 // @vitest-environment jsdom
 
-import { cleanup, render, screen } from "@testing-library/react";
+import { cleanup, render, screen, within } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { ConvertAudioControls } from "@/components/tools/convert-audio-settings";
@@ -42,6 +42,73 @@ describe("ConvertAudioControls", () => {
       expect.objectContaining({ format: "wav", bitrateKbps: 256 }),
     );
   });
+
+  it("omits sampleRate by default (preserve original)", () => {
+    const onChange = vi.fn();
+    render(<ConvertAudioControls onChange={onChange} />);
+    expect(onChange.mock.lastCall?.[0]).not.toHaveProperty("sampleRate");
+  });
+
+  it("emits the chosen sample rate on change", async () => {
+    const onChange = vi.fn();
+    render(<ConvertAudioControls onChange={onChange} />);
+    await userEvent.selectOptions(screen.getByLabelText(/sample rate/i), "44100");
+    expect(onChange).toHaveBeenLastCalledWith(expect.objectContaining({ sampleRate: 44100 }));
+  });
+
+  it("initializes sampleRate from incoming settings", () => {
+    const onChange = vi.fn();
+    render(
+      <ConvertAudioControls settings={{ format: "wav", sampleRate: 48000 }} onChange={onChange} />,
+    );
+    expect(onChange).toHaveBeenCalledWith(
+      expect.objectContaining({ format: "wav", sampleRate: 48000 }),
+    );
+  });
+
+  it("caps bitrate options for low mp3 sample rates and resets the bitrate", async () => {
+    const onChange = vi.fn();
+    render(<ConvertAudioControls onChange={onChange} />);
+    await userEvent.selectOptions(screen.getByLabelText(/sample rate/i), "8000");
+    // 192 kbps is illegal at 8 kHz (libmp3lame caps at 64); the control must
+    // reset to a legal value rather than let ffmpeg clamp silently.
+    expect(onChange).toHaveBeenLastCalledWith(
+      expect.objectContaining({ sampleRate: 8000, bitrateKbps: 64 }),
+    );
+    const bitrateSelect = screen.getByLabelText(/bitrate/i);
+    const options = [...bitrateSelect.querySelectorAll("option")].map((o) => o.value);
+    expect(options).toEqual(["32", "48", "64"]);
+
+    // Moving back to a full-range rate restores the regular options.
+    await userEvent.selectOptions(screen.getByLabelText(/sample rate/i), "44100");
+    expect(onChange).toHaveBeenLastCalledWith(
+      expect.objectContaining({ sampleRate: 44100, bitrateKbps: 192 }),
+    );
+  });
+
+  it("sanitizes an out-of-range stored sampleRate to preserve-original", () => {
+    const onChange = vi.fn();
+    render(
+      <ConvertAudioControls settings={{ format: "mp3", sampleRate: 96000 }} onChange={onChange} />,
+    );
+    // A stale or API-written value the UI cannot represent must not be emitted
+    // behind a blank select.
+    expect(onChange.mock.lastCall?.[0]).toMatchObject({ format: "mp3" });
+    expect(onChange.mock.lastCall?.[0]).not.toHaveProperty("sampleRate");
+  });
+
+  it("hides 96 kHz for mp3 and drops it when switching to mp3", async () => {
+    const onChange = vi.fn();
+    render(<ConvertAudioControls settings={{ format: "wav" }} onChange={onChange} />);
+    const rateSelect = screen.getByLabelText(/sample rate/i);
+    await userEvent.selectOptions(rateSelect, "96000");
+    expect(onChange).toHaveBeenLastCalledWith(expect.objectContaining({ sampleRate: 96000 }));
+
+    await userEvent.selectOptions(screen.getByLabelText(/output format/i), "mp3");
+    expect(onChange.mock.lastCall?.[0]).toMatchObject({ format: "mp3" });
+    expect(onChange.mock.lastCall?.[0]).not.toHaveProperty("sampleRate");
+    expect(within(rateSelect).queryByRole("option", { name: /96000/ })).toBeNull();
+  });
 });
 
 describe("TrimVideoControls", async () => {