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_en.arb

@@ -56,6 +56,8 @@
   "back": "Back",
   "previousPage": "Previous page",
   "nextPage": "Next page",
+  "toolSelectText": "Select text",
+  "actionHighlightSelection": "Highlight selection",
   "failedToOpenPdf": "Failed to open PDF:\n{error}",
   "@failedToOpenPdf": {
     "placeholders": { "error": { "type": "String" } }

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:

lib/l10n/app_localizations_en.dart

@@ -155,6 +155,12 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get nextPage => 'Next page';
 
+  @override
+  String get toolSelectText => 'Select text';
+
+  @override
+  String get actionHighlightSelection => 'Highlight selection';
+
   @override
   String failedToOpenPdf(String error) {
     return 'Failed to open PDF:\n$error';

lib/l10n/app_localizations_zh.dart

@@ -155,6 +155,12 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get nextPage => '下一页';
 
+  @override
+  String get toolSelectText => '选择文字';
+
+  @override
+  String get actionHighlightSelection => '高亮所选';
+
   @override
   String failedToOpenPdf(String error) {
     return '打开 PDF 失败：\n$error';

lib/l10n/app_zh.arb

@@ -47,6 +47,8 @@
   "back": "返回",
   "previousPage": "上一页",
   "nextPage": "下一页",
+  "toolSelectText": "选择文字",
+  "actionHighlightSelection": "高亮所选",
   "failedToOpenPdf": "打开 PDF 失败：\n{error}",
   "pdfNoPages": "PDF 没有任何页面。",
   "pageOfPages": "{current} / {total}"