Stream transcript segments to frontend as they are transcribed

Send each segment to the frontend immediately after transcription via
a new pipeline.segment IPC message, then send speaker assignments as a
batch pipeline.speaker_update message after diarization completes. This
lets the UI display segments progressively instead of waiting for the
entire pipeline to finish.

Changes:
- Add partial_segment_message and speaker_update_message IPC factories
- Add on_segment callback parameter to TranscribeService.transcribe()
- Emit partial segments and speaker updates from PipelineService.run()
- Add send_and_receive_with_progress to SidecarManager (Rust)
- Route pipeline.segment/speaker_update events in run_pipeline command
- Listen for streaming events in Svelte frontend (+page.svelte)
- Add tests for new message types, callback signature, and update logic

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/transcribe.rs

@@ -1,4 +1,5 @@
 use serde_json::{json, Value};
+use tauri::{AppHandle, Emitter};
 
 use crate::sidecar::messages::IPCMessage;
 use crate::sidecar::sidecar;
@@ -42,6 +43,7 @@ pub fn transcribe_file(
 /// Run the full transcription + diarization pipeline via the Python sidecar.
 #[tauri::command]
 pub fn run_pipeline(
+    app: AppHandle,
     file_path: String,
     model: Option<String>,
     device: Option<String>,
@@ -71,7 +73,14 @@ pub fn run_pipeline(
         }),
     );
 
-    let response = manager.send_and_receive(&msg)?;
+    let response = manager.send_and_receive_with_progress(&msg, |msg| {
+        let event_name = match msg.msg_type.as_str() {
+            "pipeline.segment" => "pipeline-segment",
+            "pipeline.speaker_update" => "pipeline-speaker-update",
+            _ => "pipeline-progress",
+        };
+        let _ = app.emit(event_name, &msg.payload);
+    })?;
 
     if response.msg_type == "error" {
         return Err(format!(