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/lib/pages/SettingsPage.svelte

@@ -1518,6 +1518,11 @@
                 label="Auto-paste into focused window"
                 description={pasteBackendsDescription}
               />
+              <Toggle
+                bind:checked={settings.transcriptionPreview}
+                label="Floating preview when Kon is unfocused"
+                description="Shows a small always-on-top window with the raw transcription as you dictate, then the final formatted text. Only opens when the main window is unfocused or hidden."
+              />
               <Toggle
                 bind:checked={settings.meetingAutoCapture}
                 label="Remind me when a meeting starts"