feat(preview): floating transcription overlay with listening→live→cleanup→final phases

Ported the best bits of OpenWhispr's TranscriptionPreviewOverlay into Kon's
window conventions. Off by default — toggle in Settings → Output → "Floating
preview when Kon is unfocused". Opens only when the main window isn't
focused at the start of a recording, so it never adds noise when the user
can already see the transcript in the main surface.

Phase state machine (src/routes/preview/+page.svelte):
- listening — pulsing dot, no text yet
- live      — animated bars + streaming raw Whisper output
- cleanup   — accent bars while the LLM cleanup pass runs
- final     — checkmark + formatted text + 4s auto-hide

Data plumbing: raw segment text is captured before post_process_segments in
live.rs (new raw_text field on LiveResultMessage) and in transcription.rs
(new raw_text in the transcription-result payload). DictationPage forwards
raw_text to the overlay via Tauri global events — preview-listening on
start, preview-append per chunk, preview-cleanup before the LLM pass,
preview-final with the formatted text, preview-hide when a run produced no
transcript (empty recording / cancel).

Window is always_on_top, skip_taskbar, focused=false so it never steals
focus from whatever the user is dictating into. open_preview_window shows
an existing hidden preview or builds it fresh; close_preview_window hides
without destroying so the next open is instant.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/windows.rs

@@ -41,6 +41,58 @@ pub async fn open_task_window(app: tauri::AppHandle) -> Result<(), String> {
     Ok(())
 }
 
+/// Open the always-on-top transcription preview window. Idempotent — an
+/// existing window is shown and focused; otherwise a new one is built.
+/// The preview is passive: it subscribes to the `transcription-result`
+/// event and the cross-window `preview-*` events that DictationPage fires
+/// as it moves through the phases of a dictation run.
+#[tauri::command]
+pub async fn open_preview_window(app: tauri::AppHandle) -> Result<(), String> {
+    if let Some(window) = app.get_webview_window("transcription-preview") {
+        window.show().map_err(|e| e.to_string())?;
+        // Intentionally NOT set_focus — the preview window is meant to
+        // appear next to whatever the user is typing into; stealing focus
+        // defeats the whole point.
+        return Ok(());
+    }
+
+    let use_native_decorations = cfg!(target_os = "linux");
+
+    let mut builder = WebviewWindowBuilder::new(
+        &app,
+        "transcription-preview",
+        WebviewUrl::App("/preview".into()),
+    )
+    .title("Kon — Preview")
+    .inner_size(420.0, 200.0)
+    .min_inner_size(360.0, 140.0)
+    .max_inner_size(520.0, 360.0)
+    .always_on_top(true)
+    .skip_taskbar(true)
+    .focused(false)
+    .decorations(use_native_decorations)
+    .resizable(true);
+
+    if let Some(script) = app.try_state::<PreferencesScript>() {
+        if !script.0.is_empty() {
+            builder = builder.initialization_script(&script.0);
+        }
+    }
+
+    builder.build().map_err(|e| e.to_string())?;
+    Ok(())
+}
+
+/// Hide the transcription preview window without destroying it so the next
+/// open is instant. Returns Ok even when no preview window exists.
+#[tauri::command]
+pub async fn close_preview_window(app: tauri::AppHandle) -> Result<(), String> {
+    if let Some(window) = app.get_webview_window("transcription-preview") {
+        window.hide().map_err(|e| e.to_string())?;
+    }
+    Ok(())
+}
+
 /// Open the transcript viewer window.
 #[tauri::command]
 pub async fn open_viewer_window(app: tauri::AppHandle) -> Result<(), String> {