fix: palm rejection, eraser sizes, PDF thumbnails, even polygon strokes (Issues #37-#41)

- #41: snapped triangles, rectangles and polygons keep the full stroke width on every
  side; corners are mitered, sharp ones bevelled (the old corner offset thinned the edges)
- #38: four eraser sizes, cycled by a button below the eraser modes and remembered in
  ui.json
- #37: a selection made with the pen's select button can be moved with the bare pen
- #39: fingers resting on the screen when the pen comes near, or touching it while the pen
  is near (and 0.7 s after), are ignored until lifted; the vendored winit ignores touches
  on the title bar meanwhile
- #40: the page picker shows imported PDF pages, rendered small in the background

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/i18n/en.rs

@@ -255,8 +255,10 @@ pub const EN: &[(&str, &str)] = &[
     ("Quell-PDF fehlt", "Source PDF missing"),
     ("Quell-PDF ist verschlüsselt", "Source PDF is encrypted"),
     ("Radieren", "Erase"),
-    ("Radierer (E oder rechte Maustaste) – darunter: ganze Striche, Teilstrich oder nur Textmarker", "Eraser (E or right mouse button) – below: whole strokes, partial stroke or highlighter only"),
+    ("Radierer (E oder rechte Maustaste) – darunter: ganze Striche, Teilstrich, nur Textmarker und die Größe", "Eraser (E or right mouse button) – below: whole strokes, partial stroke, highlighter only and the size"),
     ("Radierer (Eraser)", "Eraser"),
+    ("Radiergröße {size} mm", "Eraser size {size} mm"),
+    ("Radiergröße {size} mm – tippen für die nächste Größe", "Eraser size {size} mm – tap for the next size"),
     ("Raute (Rhombus)", "Rhombus"),
     ("Rechteck", "Rectangle"),
     ("Rechts", "Right"),