feat: pass custom words as Whisper initial_prompt instead of post-correction (#1035)

Whisper supports an `initial_prompt` parameter that biases the model's
vocabulary during transcription. For Whisper models, we now join the
user's custom words list into a comma-separated string and pass it as
the initial_prompt. This guides the model to prefer those spellings
during decoding rather than relying solely on fuzzy post-correction.

For non-Whisper engines (Parakeet, Moonshine, SenseVoice, GigaAM),
the existing apply_custom_words post-correction continues to apply
since those engines don't support prompt-based vocabulary biasing.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/managers/transcription.rs

@@ -501,6 +501,11 @@ impl TranscriptionManager {
                             let params = WhisperInferenceParams {
                                 language: whisper_language,
                                 translate: settings.translate_to_english,
+                                initial_prompt: if settings.custom_words.is_empty() {
+                                    None
+                                } else {
+                                    Some(settings.custom_words.join(", "))
+                                },
                                 ..Default::default()
                             };
 
@@ -602,8 +607,15 @@ impl TranscriptionManager {
             }
         };
 
-        // Apply word correction if custom words are configured
+        // Apply word correction if custom words are configured.
-        let corrected_result = if !settings.custom_words.is_empty() {
+        // Skip for Whisper models since custom words are already passed as initial_prompt.
+        let is_whisper = self
+            .model_manager
+            .get_model_info(&settings.selected_model)
+            .map(|info| matches!(info.engine_type, EngineType::Whisper))
+            .unwrap_or(false);
+
+        let corrected_result = if !settings.custom_words.is_empty() && !is_whisper {
             apply_custom_words(
                 &result.text,
                 &settings.custom_words,