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" } }