feat(phase9): HistoryPage .md export via save dialog

Replaces the clipboard-only path with saveTranscriptAsMarkdown. User
picks the location via the OS save dialog; cancellation leaves no side
effect, no toast, no fallback. Toast on success names the file.

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

src/lib/pages/HistoryPage.svelte

@@ -8,6 +8,10 @@
   import {
     deriveAutoTags, buildFrontmatter, buildMarkdown, normaliseTag,
   } from "$lib/utils/frontmatter.js";
+  import {
+    saveTranscriptAsMarkdown,
+    exportTranscriptsToDir,
+  } from "$lib/utils/saveMarkdown";
   import { getPreferences } from "$lib/stores/preferences.svelte.js";
   import { clampTextLines, measurePreWrap } from "$lib/utils/textMeasure.js";
   import { bodyPretextLineHeight, pretextFontShorthand } from "$lib/utils/accessibilityTypography.js";
@@ -341,15 +345,7 @@
   }
 
   async function exportMarkdown(item) {
-    const md = buildMarkdown(item);
-    try {
-      await navigator.clipboard.writeText(md);
-    } catch {
-      try {
-        await invoke("copy_to_clipboard", { text: md });
-      } catch {}
-    }
-    toasts.info("Markdown copied to clipboard — paste into Obsidian or save as .md");
+    await saveTranscriptAsMarkdown(item);
   }
 
   async function openEditor(item) {