feat: import Saber notebooks (Issue #47)

The import now also accepts Saber files: .sbn2 (BSON) with its asset
files next to it, .sba archives (zip with main.sbn2 and assets) and the
old JSON .sbn, including the layout before format version 8 with
strokes outside the pages. Taken over: strokes with pen pressure and
perfect_freehand's thinning, highlighters, circle and rectangle shapes,
typed text (Quill delta as one text field), images (SVG rasterized),
background images and PDF pages as PDF backgrounds, paper patterns.

BSON and zip are read by small built-in readers, so no new
dependencies. The SVG rasterizer moved to src/import.rs.

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

src/import.rs

@@ -1,6 +1,6 @@
 //! Import of other formats as a new notebook (Issue #47): PDF (`storage`), Xournal++
-//! (`xopp`) and Rnote (`rnote`). This module holds what the importers share: building
-//! strokes, texts and images in the stored format and creating the notebook.
+//! (`xopp`), Rnote (`rnote`) and Saber (`saber`). This module holds what the importers
+//! share: building strokes, texts and images in the stored format and creating the notebook.
 
 use std::io::Read;
 use std::path::{Path, PathBuf};
@@ -37,9 +37,10 @@ pub fn is_importable_path(path: &Path) -> bool {
     crate::file_picker::is_pdf_path(path)
         || crate::xopp::is_xopp_path(path)
         || crate::rnote::is_rnote_path(path)
+        || crate::saber::is_saber_path(path)
 }
 
-/// Imports a PDF, Xournal++ or Rnote file as a new notebook titled `title`.
+/// Imports a PDF, Xournal++, Rnote or Saber file as a new notebook titled `title`.
 pub fn import_file(
     base_dir: &Path,
     title: &str,
@@ -49,6 +50,8 @@ pub fn import_file(
         crate::xopp::create_notebook_from_xopp(base_dir, title, path)
     } else if crate::rnote::is_rnote_path(path) {
         crate::rnote::create_notebook_from_rnote(base_dir, title, path)
+    } else if crate::saber::is_saber_path(path) {
+        crate::saber::create_notebook_from_saber(base_dir, title, path)
     } else {
         let bytes = std::fs::read(path).map_err(|e| e.to_string())?;
         storage::create_notebook_from_pdf(base_dir, title, bytes)
@@ -228,6 +231,37 @@ pub fn image_stroke(id: u64, asset: String, corners: [glam::Vec2; 4]) -> StrokeD
     }
 }
 
+/// Premultiplied RGBA (Rnote, tiny-skia) to straight RGBA.
+pub fn unpremultiply(pixels: &mut [u8]) {
+    for px in pixels.as_chunks_mut::<4>().0 {
+        let a = px[3] as u32;
+        if a > 0 && a < 255 {
+            for c in &mut px[..3] {
+                *c = ((*c as u32 * 255 + a / 2) / a).min(255) as u8;
+            }
+        }
+    }
+}
+
+/// Rasterizes an SVG for a box of `size` page units (about 250 dpi).
+pub fn rasterize_svg(svg: &str, size: glam::Vec2) -> Option<(u32, u32, Vec<u8>)> {
+    use resvg::{tiny_skia, usvg};
+    let tree = usvg::Tree::from_str(svg, &usvg::Options::default()).ok()?;
+    let max = crate::assets::MAX_IMPORT_DIMENSION as f32;
+    let px_per_unit = 250.0 / 254.0;
+    let width = (size.x * px_per_unit).round().clamp(1.0, max) as u32;
+    let height = (size.y * px_per_unit).round().clamp(1.0, max) as u32;
+    let mut pixmap = tiny_skia::Pixmap::new(width, height)?;
+    let transform = tiny_skia::Transform::from_scale(
+        width as f32 / tree.size().width(),
+        height as f32 / tree.size().height(),
+    );
+    resvg::render(&tree, transform, &mut pixmap.as_mut());
+    let mut pixels = pixmap.take();
+    unpremultiply(&mut pixels);
+    Some((width, height, pixels))
+}
+
 fn corner_samples(corners: [glam::Vec2; 4]) -> Vec<SampleData> {
     corners
         .iter()