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/routes/preview/+layout@.svelte

@@ -0,0 +1,68 @@
+<script lang="ts">
+  // @ts-nocheck
+  import "../../app.css";
+  import { onDestroy, onMount } from "svelte";
+  import { getCurrentWindow } from "@tauri-apps/api/window";
+  import { listen } from "@tauri-apps/api/event";
+  import { settings } from "$lib/stores/page.svelte.js";
+  import {
+    getPreferences,
+    updatePreferences,
+    applyExternalPreferences,
+    PREFERENCES_CHANGED_EVENT,
+  } from "$lib/stores/preferences.svelte.js";
+
+  let { children } = $props();
+  let unlistenPrefs = null;
+
+  const prefs = getPreferences();
+
+  // Keep transcript-editor theme sync trick: legacy settings → preferences
+  $effect(() => {
+    const legacyTheme = settings.theme;
+    const mapped = legacyTheme === "Light" ? "light" : legacyTheme === "Dark" ? "dark" : "system";
+    if (prefs.theme !== mapped) {
+      updatePreferences({ theme: mapped });
+    }
+  });
+
+  if (typeof window !== "undefined") {
+    window.addEventListener("storage", (event) => {
+      if (event.key === "kon_settings" && event.newValue) {
+        try { Object.assign(settings, JSON.parse(event.newValue)); } catch {}
+      }
+    });
+  }
+
+  onMount(async () => {
+    try {
+      let ownLabel = null;
+      try { ownLabel = getCurrentWindow().label; } catch {}
+      unlistenPrefs = await listen(PREFERENCES_CHANGED_EVENT, (event) => {
+        const payload = event?.payload;
+        if (!payload || payload.source === ownLabel) return;
+        applyExternalPreferences(payload.prefs);
+      });
+    } catch {}
+  });
+
+  onDestroy(() => {
+    if (unlistenPrefs) unlistenPrefs();
+  });
+
+  // Escape closes the preview without destroying it — the next dictation
+  // reopens it instantly via open_preview_window.
+  function handleKeydown(event) {
+    if (event.key === "Escape") {
+      getCurrentWindow().hide().catch(() => {});
+    }
+  }
+</script>
+
+<svelte:window onkeydown={handleKeydown} />
+
+<div class="h-screen w-screen overflow-hidden grain border border-border shadow-xl flex flex-col">
+  <div class="flex-1 min-h-0 overflow-hidden">
+    {@render children()}
+  </div>
+</div>