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/pdf_doc.rs

@@ -21,6 +21,8 @@ use crate::t;
 pub const DISPLAY_RENDER_PIXELS: u32 = 2560;
 /// Längste Kante beim starken Hineinzoomen (A4 ≈ 440 dpi).
 pub const ZOOMED_RENDER_PIXELS: u32 = 5120;
+/// Longest edge of a page picker thumbnail (sharp at display scale 2).
+pub const THUMBNAIL_RENDER_PIXELS: u32 = 256;
 
 /// Texturschlüssel einer PDF-Seite: `<asset>#<seite>`, gerendert mit mehr als
 /// [`DISPLAY_RENDER_PIXELS`] als `<asset>#<seite>@<pixel>`. Liefert Asset, Seite und Pixel.