Add HuggingFace token setting for speaker detection

- Add "Speakers" tab in Settings with HF token input field
- Include step-by-step instructions for obtaining the token
- Pass hf_token from settings through Rust → Python pipeline → diarize
- Token can also be set via HF_TOKEN environment variable as fallback
- Move skip_diarization checkbox to Speakers tab

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/stores/settings.ts

@@ -14,6 +14,7 @@ export interface AppSettings {
   transcription_device: string;
   transcription_language: string;
   skip_diarization: boolean;
+  hf_token: string;
 }
 
 const defaults: AppSettings = {
@@ -29,6 +30,7 @@ const defaults: AppSettings = {
   transcription_device: 'cpu',
   transcription_language: '',
   skip_diarization: false,
+  hf_token: '',
 };
 
 export const settings = writable<AppSettings>({ ...defaults });