Phase 4: Export to SRT, WebVTT, ASS, plain text, and Markdown

- Implement ExportService using pysubs2 for caption formats (SRT, VTT, ASS)
  and custom formatters for plain text and Markdown
- SRT exports with [Speaker]: prefix, WebVTT with <v Speaker> voice tags,
  ASS with color-coded speaker styles
- Plain text groups by speaker with labels, Markdown adds timestamps
- Add export.start IPC handler and export_transcript Tauri command
- Add export dropdown menu in header (appears after transcription)
- Uses native save dialog for output file selection
- Add pysubs2 dependency
- Tests: 30 Python (6 export tests), 6 Rust, 0 Svelte errors

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

src-tauri/src/commands/export.rs

@@ -1,2 +1,49 @@
-// Export commands — trigger caption/text export via Python sidecar
-// TODO: Implement when export service is built
+use serde_json::{json, Value};
+
+use crate::sidecar::messages::IPCMessage;
+use crate::sidecar::SidecarManager;
+
+/// Export transcript to caption/text format via the Python sidecar.
+#[tauri::command]
+pub fn export_transcript(
+    segments: Value,
+    speakers: Value,
+    format: String,
+    output_path: String,
+    title: Option<String>,
+) -> Result<Value, String> {
+    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,
+        "export.start",
+        json!({
+            "segments": segments,
+            "speakers": speakers,
+            "format": format,
+            "output_path": output_path,
+            "title": title.unwrap_or_default(),
+        }),
+    );
+
+    let response = manager.send_and_receive(&msg)?;
+
+    if response.msg_type == "error" {
+        return Err(format!(
+            "Export error: {}",
+            response.payload.get("message").and_then(|v| v.as_str()).unwrap_or("unknown")
+        ));
+    }
+
+    Ok(response.payload)
+}