feat(pdf): rebuild editor on vector PdfViewer

Replace the single-page PdfPageView bitmap with a pdfrx
PdfViewer: real vector text, continuous scroll, native
pinch-zoom (no custom zoom solver, so no zoom-jump here).

Ink is glued per-page via pageOverlaysBuilder; the pen is
captured at the viewer level by PenCaptureRegion while touch
falls through to scroll / pinch / text-select.

Add select-text -> highlight via PdfTextSelectionParams: the
selection's fragment rects are stored as normalized page rects
and drawn under the ink. In-memory only for now.

Per-page persistence, undo/redo, tools, colors, thumbnails and
pen settings are reused verbatim. analyze clean, 270 tests green.

lib/l10n/app_localizations.dart

@@ -380,6 +380,18 @@ abstract class AppLocalizations {
   /// **'Next page'**
   String get nextPage;
 
+  /// No description provided for @toolSelectText.
+  ///
+  /// In en, this message translates to:
+  /// **'Select text'**
+  String get toolSelectText;
+
+  /// No description provided for @actionHighlightSelection.
+  ///
+  /// In en, this message translates to:
+  /// **'Highlight selection'**
+  String get actionHighlightSelection;
+
   /// No description provided for @failedToOpenPdf.
   ///
   /// In en, this message translates to: