fix(viewer): hand off transcript ID only, fetch row from SQLite on mount

HistoryPage previously serialised the full TranscriptDto — text, segments,
manual + LLM tags, audio path — into `localStorage["kon_viewer_item"]` so
the viewer window could pick it up on mount. On a multi-hour transcript
that's MB-scale of user voice content sitting in storage that any
same-origin script in any open Kon window can read.

Hand off only `{ id }` (and a timestamp on re-saves). The viewer fetches
the canonical row from SQLite via the existing `get_transcript` Tauri
command and hydrates via the now-exported `mapTranscriptRow`. Cross-window
sync via the `storage` event still works — the receiving window re-fetches
on event instead of trusting the payload.

- HistoryPage `openViewer` + `openEditor`: write `{ id }` only.
- viewer `onMount` + `handleStorageChange`: route through new
  `loadFromHandoff` which calls `invoke("get_transcript", { id })`.
- viewer `saveItemToHistory`: re-stamp localStorage with `{ id, stamp }`
  to retrigger the storage event in sibling windows without leaking
  content.
- `mapTranscriptRow` exported from page.svelte.ts for the viewer's use.

Backward-compatible at the parse layer: the `{ id }` shape extracts cleanly
from a stale full-DTO payload (TranscriptEntry already carries `id` at top
level), so a session that survives the upgrade picks up the new path on
next handoff without manual cleanup.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

src/routes/viewer/+page.svelte

@@ -2,13 +2,13 @@
   import { onMount, onDestroy, tick } from "svelte";
   import { getCurrentWindow } from "@tauri-apps/api/window";
   import { convertFileSrc, invoke } from "@tauri-apps/api/core";
-  import type { TranscriptEntry, ViewerSegment } from "$lib/types/app";
+  import type { TranscriptDto, TranscriptEntry, ViewerSegment } from "$lib/types/app";
   import SpeakerButton from "$lib/components/SpeakerButton.svelte";
   import { formatTime, formatDuration } from "$lib/utils/time.js";
   import { PLAYBACK_SPEEDS } from "$lib/utils/constants.js";
   import { errorMessage } from "$lib/utils/errors.js";
   import { parseStoredJson } from "$lib/utils/storage.js";
-  import { saveTranscriptMeta } from "$lib/stores/page.svelte.js";
+  import { mapTranscriptRow, saveTranscriptMeta } from "$lib/stores/page.svelte.js";
   import { toasts } from "$lib/stores/toasts.svelte.ts";
   import { DEFAULT_PROFILE_ID } from "$lib/stores/profiles.svelte.ts";
 
@@ -71,8 +71,28 @@
     buildAudio(nextItem?.audioPath ?? null);
   }
 
+  // Hydrate the viewer from a transcript ID handed off via localStorage.
+  // The full row is fetched from SQLite via `get_transcript` so transcript
+  // text never lives in localStorage where any same-origin script could
+  // read it.
+  async function loadFromHandoff(raw: string | null) {
+    const handoff = parseStoredJson<{ id?: unknown }>(raw);
+    const id = handoff?.id != null ? String(handoff.id) : "";
+    if (!id) {
+      loadViewerItem(null);
+      return;
+    }
+    try {
+      const dto = await invoke<TranscriptDto | null>("get_transcript", { id });
+      loadViewerItem(dto ? mapTranscriptRow(dto) : null);
+    } catch (err) {
+      console.warn("viewer get_transcript failed", errorMessage(err));
+      loadViewerItem(null);
+    }
+  }
+
   onMount(() => {
-    loadViewerItem(parseStoredJson<TranscriptEntry>(localStorage.getItem("kon_viewer_item")));
+    loadFromHandoff(localStorage.getItem("kon_viewer_item"));
     const mode = localStorage.getItem("kon_viewer_mode");
     if (mode === "edit" || mode === "view") viewerMode = mode;
 
@@ -120,7 +140,7 @@
 
   function handleStorageChange(e: StorageEvent) {
     if (e.key === "kon_viewer_item" && e.newValue) {
-      loadViewerItem(parseStoredJson<TranscriptEntry>(e.newValue));
+      void loadFromHandoff(e.newValue);
     }
   }
 
@@ -289,14 +309,17 @@
   // store's `saveTranscriptMeta` helper — callers in this file wire those
   // up alongside the plain-text update below.
   //
-  // The cross-window `kon_viewer_item` localStorage payload stays in sync so
-  // a second viewer window hydrating from it reflects the latest edit.
+  // We re-stamp the localStorage handoff with the same ID so a sibling
+  // viewer window receives a `storage` event and re-fetches from SQLite.
+  // The handoff intentionally contains no transcript content — see
+  // `loadFromHandoff` for the rationale.
   function saveItemToHistory() {
     if (!item) return;
-    // Update the cross-window payload only. The old localStorage history
-    // cache was removed as part of the SQLite-canonical refactor.
     try {
-      localStorage.setItem("kon_viewer_item", JSON.stringify(item));
+      localStorage.setItem(
+        "kon_viewer_item",
+        JSON.stringify({ id: String(item.id), stamp: Date.now() }),
+      );
     } catch {}
     // Best-effort SQLite persistence. Silently ignored in browser preview
     // (no Tauri runtime) — matches the main-window store's behaviour.