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>

vendor/winit/README.ink-notes.md

@@ -11,6 +11,9 @@ docs of the original crate were removed.
   are now passed to the frame like a left click: close, minimize and maximize work with a
   finger, and dragging the title bar moves the window (Issue #36). All changes are marked with
   `ink-notes patch`.
+- `src/platform/wayland.rs`: `ignore_decoration_touches_until` lets ink-notes ignore touches on
+  the title bar while the pen is near the screen, so a resting palm does not move the window
+  (Issue #39).
 - `Cargo.toml`: `[lints.rust] warnings = "allow"`, because a path dependency is not built with
   capped lints like a crates.io dependency.