feat(pdf): typed-text tool (Windows-Ink friendly)

Add a text-annotation tool to the PDF editor. With the text tool a
pen-tap, or a mouse double-click, drops a text box at that normalized
page point and focuses a real Flutter TextField — so the OS IME and the
Windows-Ink handwriting panel feed it (device-validated). Tapping an
existing box re-opens it; clearing it deletes it.

- SidecarText {nx, ny, text, fontSize (page-relative), color} per page,
  glued under zoom; stored in the sidecar `texts` field (back-compat
  missing -> none), saved via scheduleTextsSave and loaded on open.
- Rendered in pageOverlaysBuilder at the scaled position.

PDF editor only for now (note text later). analyze clean, 397 tests.

lib/l10n/app_en.arb

@@ -91,6 +91,8 @@
   "actionHighlightSelection": "Highlight selection",
   "toolRemoveHighlight": "Remove highlight (tap a highlight)",
   "toolPlaceScratchLink": "Place scratch link",
+  "toolText": "Text (tap or double-click to add)",
+  "textPlaceholder": "Type…",
   "scratchLinkDeleteTitle": "Delete scratch link?",
   "scratchLinkDeleteBody": "This removes the anchor and its private scratchpad.",
   "toolAddBookmark": "Add bookmark (here or at selection)",