feat(editor): transcribe embedded audio blocks

Add a gated "Transcribe" action to the audio block's bubble menu so an
already-embedded audio file can be transcribed (previously only live
microphone dictation was supported). The button fetches the embedded
file, normalizes its MIME type to the STT whitelist, reuses the existing
POST /ai-chat/transcribe endpoint, and inserts the result as a paragraph
right below the audio block.

- Mount the previously-unwired AudioMenu in page-editor (edit mode only),
  which also surfaces the existing Download/Delete actions for audio.
- Gate the Transcribe button on settings.ai.dictation; show a spinner and
  block double-submits while transcribing; map errors like the mic hook.
- Disambiguate duplicate-src blocks by re-scanning the doc and inserting
  after the audio node closest to the originally selected one.
- Add i18n keys (en-US, ru-RU): Transcribe, Transcribing…, No speech
  detected, plus ru-RU translations for the transcription error messages.

apps/client/src/features/editor/page-editor.tsx

@@ -49,6 +49,7 @@ import { TableHandlesLayer } from "@/features/editor/components/table/handle/tab
 import ImageMenu from "@/features/editor/components/image/image-menu.tsx";
 import CalloutMenu from "@/features/editor/components/callout/callout-menu.tsx";
 import VideoMenu from "@/features/editor/components/video/video-menu.tsx";
+import AudioMenu from "@/features/editor/components/audio/audio-menu.tsx";
 import PdfMenu from "@/features/editor/components/pdf/pdf-menu.tsx";
 import SubpagesMenu from "@/features/editor/components/subpages/subpages-menu.tsx";
 import {
@@ -461,6 +462,7 @@ export default function PageEditor({
                 <TableHandlesLayer editor={editor} />
                 <ImageMenu editor={editor} />
                 <VideoMenu editor={editor} />
+                <AudioMenu editor={editor} />
                 <PdfMenu editor={editor} />
                 <CalloutMenu editor={editor} />
                 <SubpagesMenu editor={editor} />