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.
Replace the ad-hoc tool palette with a shared tool system
(EditorToolKind) across the PDF, note and slide editors, and add
the core writing tools.
- Multiple brushes, each remembering its OWN color (rnote-style):
selecting a brush restores its color, changing color updates only
that brush, and each brush button shows its current color.
- Select tool: tap-select a committed stroke, drag to move it,
delete it — persisted and undoable.
- Shape tool: line / rectangle / ellipse / arrow, drawn with a live
preview and committed as generated PenStrokes (shape_geometry.dart)
so they reuse stroke rendering, erase, persistence and undo.
- Highlighter + eraser fold into the same tool system.
Text/bookmark/search+OCR/backgrounds/Windows-Ink are later batches
(TODO). Brush opacity still deferred. analyze clean, 302 tests.
