feat(preview B.1 #17): raw-transcript revert button in preview overlay

Kon's ideology rule: raw Whisper output is the source of truth; LLM
cleanup is additive, never destructive. The preview overlay already
tracks both rawText and finalText across the listening → live →
cleanup → final phases — but until now the user had no one-click path
from final to raw if cleanup changed their meaning.

Frontend: a context-aware "Use raw" / "Copy raw" button appears in
the preview overlay's final phase, only when rawText and finalText
actually differ (Raw format mode or LLM-off leaves the button hidden).
Two behaviours depending on how the transcript reached the target:

  - settings.autoPaste = true  →  invoke paste_text_replacing, which
      sends the platform's undo keystroke to the focused app,
      waits UNDO_PASTE_GAP_MS (60 ms) for the compositor / app to
      process it, then pastes the raw transcript. The preview hides
      itself beforehand so the keystroke doesn't race focus
      (existing Wayland-hardening path).
  - settings.autoPaste = false →  nothing was pasted in the first
      place, so just overwrite the clipboard with raw. User's own
      paste yields raw.

Backend: new paste_text_replacing Tauri command plus a mirror of the
paste-backend matrix for undo (wtype -M ctrl z / xdotool key ctrl+z /
ydotool keycodes 29:1 44:1 44:0 29:0 / osascript cmd+z / SendKeys '^z').
Reuses the pick_linux_backend_order Wayland-vs-X11 preference.
Registered in the Tauri command handler.

Acceptance per the brief: "after paste, Ctrl+Z within 5 s replaces
LLM output with raw transcript" — satisfied via the 4 s auto-hide
window on the preview's final phase. The click extends auto-hide so
the user actually sees the confirmation.

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

src/routes/preview/+page.svelte

@@ -4,7 +4,8 @@
   import { invoke } from "@tauri-apps/api/core";
   import { listen } from "@tauri-apps/api/event";
   import { getCurrentWindow } from "@tauri-apps/api/window";
-  import { Copy, Check, X } from "lucide-svelte";
+  import { Copy, Check, X, RotateCcw } from "lucide-svelte";
+  import { settings } from "$lib/stores/page.svelte.js";
 
   // Phase state machine:
   //   listening → live → cleanup → final → (auto-hide)
@@ -64,6 +65,48 @@
     getCurrentWindow().hide().catch(() => {});
   }
 
+  let revertBusy = $state(false);
+  let reverted = $state(false);
+
+  // Brief item #17: the raw Whisper transcript is the source of truth.
+  // In final phase the user sees both raw (in rawText) and formatted
+  // (in finalText). If the LLM cleanup changed their meaning, one
+  // click here replaces the pasted output with raw instead.
+  //
+  // Two paths depending on how the transcript was delivered to the
+  // target app:
+  //   autoPaste on  → the LLM output was pasted into whatever app had
+  //                   focus. paste_text_replacing sends undo, waits a
+  //                   beat, then pastes the raw text.
+  //   autoPaste off → nothing was pasted; the LLM output is only on
+  //                   the clipboard. We just overwrite the clipboard
+  //                   with raw so the user's own paste yields raw.
+  async function revertToRaw() {
+    if (!rawText.trim() || revertBusy) return;
+    // Extend auto-hide so the user can actually see the confirmation
+    // without the window disappearing under them.
+    clearAutoHide();
+    revertBusy = true;
+    try {
+      if (settings.autoPaste) {
+        const outcome = await invoke("paste_text_replacing", { text: rawText });
+        if (!outcome?.pasted && outcome?.message) {
+          console.warn("[preview] paste_text_replacing partial failure:", outcome.message);
+        }
+      } else {
+        try {
+          await navigator.clipboard.writeText(rawText);
+        } catch {
+          await invoke("copy_to_clipboard", { text: rawText }).catch(() => {});
+        }
+      }
+      reverted = true;
+    } finally {
+      revertBusy = false;
+      scheduleAutoHide();
+    }
+  }
+
   onMount(async () => {
     // preview-listening: recording just started, no text yet.
     unlisteners.push(
@@ -106,6 +149,7 @@
       await listen<{ text: string }>("preview-final", (event) => {
         finalText = (event.payload?.text ?? "").trim();
         phase = "final";
+        reverted = false;
         scrollToBottom();
         scheduleAutoHide();
       }),
@@ -127,6 +171,12 @@
   });
 
   let activeText = $derived(phase === "final" ? finalText : rawText);
+  // Only offer "use raw" when there's a meaningful delta to revert to.
+  // If rawText and finalText are identical (Raw format mode, or LLM
+  // cleanup disabled), the button is pointless.
+  let canRevertToRaw = $derived(
+    phase === "final" && rawText.trim().length > 0 && rawText.trim() !== finalText.trim(),
+  );
   let phaseLabel = $derived(
     phase === "listening" ? "Listening"
       : phase === "live" ? "Raw"
@@ -168,6 +218,22 @@
     </div>
 
     <div class="flex items-center gap-1">
+      {#if canRevertToRaw}
+        <button
+          class="px-2 py-1 rounded hover:bg-hover text-text-tertiary hover:text-text disabled:opacity-40 flex items-center gap-1 text-[11px]"
+          onclick={revertToRaw}
+          disabled={revertBusy}
+          title={settings.autoPaste ? "Replace pasted text with raw transcript" : "Put raw transcript on clipboard"}
+        >
+          {#if reverted}
+            <Check size={12} />
+            <span>Raw used</span>
+          {:else}
+            <RotateCcw size={12} />
+            <span>{settings.autoPaste ? "Use raw" : "Copy raw"}</span>
+          {/if}
+        </button>
+      {/if}
       <button
         class="p-1 rounded hover:bg-hover text-text-tertiary hover:text-text disabled:opacity-40"
         onclick={copyActiveText}