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/live.rs

@@ -112,6 +112,11 @@ pub struct LiveResultMessage {
     pub language: String,
     pub inference_ms: u64,
     pub segments: Vec<Segment>,
+    /// Concatenated text BEFORE post-processing (no filler removal, no
+    /// British conversion, no LLM cleanup). Used by the transcription
+    /// preview overlay so the user can see raw Whisper output as it
+    /// streams in.
+    pub raw_text: String,
 }
 
 #[derive(Debug, Clone, Serialize)]
@@ -616,6 +621,14 @@ fn poll_inference(
         Ok(Ok(timed)) => {
             let mut segments: Vec<Segment> = timed.transcript.segments().to_vec();
             trim_overlap_segments(&mut segments, task.trim_before_secs);
+            // Capture raw text BEFORE any post-processing so the preview
+            // overlay can show what Whisper actually returned.
+            let raw_text = segments
+                .iter()
+                .map(|segment| segment.text.trim())
+                .filter(|segment| !segment.is_empty())
+                .collect::<Vec<_>>()
+                .join(" ");
             post_process_segments(
                 &mut segments,
                 &PostProcessOptions {
@@ -645,6 +658,7 @@ fn poll_inference(
                     language: timed.transcript.language().to_string(),
                     inference_ms: timed.inference_ms,
                     segments,
+                    raw_text,
                 })
                 .map_err(|e| e.to_string())?;
             remember_recent_segments(recent_segments, &delivered_segments, chunk_start_secs);