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/view.rs

@@ -415,8 +415,8 @@ fn layout_context_menu_overlay(name: &str) -> Element<'_, Message> {
 
     let menu = container(
         column![
-            item("Yeniden Adlandır", Message::LayoutRenameStart(rename_name)),
-            item("Sil", Message::LayoutDelete(delete_name)),
+            item("Rename", Message::LayoutRenameStart(rename_name)),
+            item("Delete", Message::LayoutDelete(delete_name)),
         ]
         .spacing(0)
         .width(160),