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

packages/shared/src/i18n/ru.ts

@@ -2504,6 +2504,8 @@ export const ru: TranslationKeys = {
     "convert-audio": {
       format: "Формат вывода",
       bitrate: "Битрейт (kbps)",
+      sampleRate: "Частота дискретизации (Hz)",
+      sampleRatePreserve: "Сохранить исходную",
       submit: "Конвертировать",
       submitBatch: "Конвертировать ({count} файлов)",
       progressLabel: "Конвертация",