Fix: paper limits, Cohen-Sutherland clipping, frozen layers UI, PDF improvements

Paper limits:
- Change A4 fallback from 12×9 to 297×210 mm in paper_limits()

Context menu:
- Translate layout context menu labels to English (Rename / Delete)

Viewport clipping:
- Replace AABB-only culling with proper Cohen-Sutherland line clipping
  in viewport_content_wires(); each projected segment is now clipped at
  the exact viewport boundary instead of being included or discarded whole
- Add cs_clip() and clip_polyline_to_rect() free functions in scene/mod.rs

Frozen layers per-viewport UI:
- Inject a "Frozen Layers" EditText property in the Properties panel for
  viewport entities (layer names resolved from handles at display time)
- Handle "frozen_layers" in PropGeomCommit: parse comma-separated names,
  resolve to layer handles, update vp.frozen_layers in-place

PDF export improvements:
- Add offset_x/offset_y params to export_pdf() for model-space normalization:
  coordinates are shifted so the drawing origin maps to the page origin
- Model space export computes extents via model_space_extents() (now public)
  and adds a 5 % margin; falls back to A4 landscape for empty drawings
- Skip __paper_boundary__ wire in PDF (white background covers it)
- Map yellow (active viewport border) → black and cyan (vp border) → dark
  blue for print-friendly output

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/properties.rs

@@ -55,6 +55,29 @@ impl H7CAD {
                     let group_names = self.tabs[i].scene.group_names_for_entity(handle);
                     let mut sections =
                         dispatch::properties_sectioned(handle, entity, &text_style_names);
+
+                    // Inject frozen-layer property for viewports (requires doc access).
+                    if let acadrust::EntityType::Viewport(vp) = entity {
+                        let frozen_names: Vec<String> = vp
+                            .frozen_layers
+                            .iter()
+                            .filter_map(|&h| {
+                                self.tabs[i].scene.document.layers.iter()
+                                    .find(|l| l.handle == h)
+                                    .map(|l| l.name.clone())
+                            })
+                            .collect();
+                        if let Some(geom) = sections.last_mut() {
+                            geom.props.push(crate::scene::object::Property {
+                                label: "Frozen Layers".to_string(),
+                                field: "frozen_layers",
+                                value: crate::scene::object::PropValue::EditText(
+                                    frozen_names.join(", "),
+                                ),
+                            });
+                        }
+                    }
+
                     if !group_names.is_empty() {
                         let label = group_names.join(", ");
                         if let Some(general) = sections.first_mut() {