feat(search): index PDF text, OCR scanned PDFs on import

Search now covers handwriting, the PDF text layer, AND scanned
(rasterized) PDFs.

- PdfTextIndexer runs at import: sums the embedded text layer across
  pages; if present it stores that as the document body, otherwise the
  PDF is rasterized and its rendered pages are OCR'd in the background.
  The result lands in the sidecar `pageText` field (distinct from
  `ocrText`, the handwriting OCR). Idempotent (skips a sidecar that
  already has pageText); degrades gracefully with no OCR engine.
- pdfrx_page_text_source abstracts text/render so it's testable.
- VaultSearchIndex now harvests title + typed text + handwriting OCR +
  PDF pageText, so search finds notes, typed PDFs and scanned PDFs.

analyze clean, 409 tests green.

lib/editor/persistence/sidecar_repository.dart

@@ -183,6 +183,18 @@ class SidecarRepository {
   /// The OCR text loaded from the sidecar, or null.
   String? get loadedOcrText => _sidecar.ocrText;
 
+  /// Replace the document-body search text (PDF embedded text layer, or
+  /// background OCR of a rasterized PDF — see [PdfTextIndexer]) and schedule a
+  /// save. No-op if unchanged. An empty string is normalized to null.
+  void schedulePageTextSave(String? pageText) {
+    final next = (pageText != null && pageText.isEmpty) ? null : pageText;
+    if (_sidecar.pageText == next) return;
+    _replace(pageText: next, clearPageText: next == null);
+  }
+
+  /// The document-body search text loaded from the sidecar, or null.
+  String? get loadedPageText => _sidecar.pageText;
+
   /// Replace the committed strokes for [pageIndex] and schedule a save.
   void scheduleStrokeSave(int pageIndex, List<EditorStroke> strokes) {
     final next = Map<int, List<EditorStroke>>.from(_sidecar.strokes);
@@ -321,6 +333,8 @@ class SidecarRepository {
     List<SidecarScratchLink>? scratchLinks,
     String? ocrText,
     bool clearOcrText = false,
+    String? pageText,
+    bool clearPageText = false,
     String? background,
   }) {
     if (_disposed) return;
@@ -339,6 +353,7 @@ class SidecarRepository {
       bookmarks: bookmarks ?? _sidecar.bookmarks,
       scratchLinks: scratchLinks ?? _sidecar.scratchLinks,
       ocrText: clearOcrText ? null : (ocrText ?? _sidecar.ocrText),
+      pageText: clearPageText ? null : (pageText ?? _sidecar.pageText),
       background: background ?? _sidecar.background,
     );
     _timer?.cancel();