Phase 2: Core transcription pipeline and audio playback

- Implement faster-whisper TranscribeService with word-level timestamps,
  progress reporting, and hardware auto-detection
- Wire up Rust SidecarManager for Python process lifecycle (spawn, IPC, shutdown)
- Add transcribe_file Tauri command bridging frontend to Python sidecar
- Integrate wavesurfer.js WaveformPlayer with play/pause, skip, seek controls
- Build TranscriptEditor with word-level click-to-seek and active highlighting
- Connect file import flow: prompt → asset load → transcribe → display
- Add typed tauri-bridge service with TranscriptionResult interface
- Add Python tests for hardware detection and transcription result formatting

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

src-tauri/src/commands/transcribe.rs

@@ -1,2 +1,52 @@
-// Transcription commands — start/stop/monitor transcription via Python sidecar
-// TODO: Implement when sidecar IPC is connected
+use serde_json::{json, Value};
+
+use crate::sidecar::messages::IPCMessage;
+use crate::sidecar::SidecarManager;
+
+/// Start transcription of an audio file via the Python sidecar.
+///
+/// This is a blocking command — it starts the sidecar if needed,
+/// sends the transcribe request, and waits for the result.
+#[tauri::command]
+pub fn transcribe_file(
+    file_path: String,
+    model: Option<String>,
+    device: Option<String>,
+    language: Option<String>,
+) -> Result<Value, String> {
+    // Determine Python sidecar path (relative to app)
+    let python_path = std::env::current_dir()
+        .map_err(|e| e.to_string())?
+        .join("../python")
+        .canonicalize()
+        .map_err(|e| format!("Cannot find python directory: {e}"))?;
+
+    let python_path_str = python_path.to_string_lossy().to_string();
+
+    let manager = SidecarManager::new();
+    manager.start(&python_path_str)?;
+
+    let request_id = uuid::Uuid::new_v4().to_string();
+    let msg = IPCMessage::new(
+        &request_id,
+        "transcribe.start",
+        json!({
+            "file": file_path,
+            "model": model.unwrap_or_else(|| "base".to_string()),
+            "device": device.unwrap_or_else(|| "cpu".to_string()),
+            "compute_type": "int8",
+            "language": language,
+        }),
+    );
+
+    let response = manager.send_and_receive(&msg)?;
+
+    if response.msg_type == "error" {
+        return Err(format!(
+            "Transcription error: {}",
+            response.payload.get("message").and_then(|v| v.as_str()).unwrap_or("unknown")
+        ));
+    }
+
+    Ok(response.payload)
+}