feat(web): implement complete workspace with themes, tabs, sidebar, and mobile

Transform CalcText from a single-document calculator into a full workspace
application with multi-document support, theming, and responsive mobile experience.

- Theme system: 5 presets (Light, Dark, Matrix, Midnight, Warm) + accent colors
- Document model with localStorage persistence and auto-save
- Tab bar with keyboard shortcuts (Ctrl+N/W/Tab/1-9), rename, close
- Sidebar with search, recent, favorites, folders, templates, drag-and-drop
- 5 templates: Budget, Invoice, Unit Converter, Trip Planner, Loan Calculator
- Status bar with cursor position, engine status, dedication to Igor Cassel
- Results panel: type-specific colors, click-to-copy, error hints
- Format toolbar: H, B, I, //, color labels with live preview toggle
- Syntax highlighting using theme CSS variables
- Error hover tooltips
- Mobile: bottom results tray, sidebar drawer, touch targets, safe areas
- Docker multi-stage build (Rust WASM + Vite + Nginx)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

calcpad-engine/tests/sheet_scenarios.rs

@@ -123,7 +123,7 @@ fn sheet_with_comments() {
         ],
         &mut ctx,
     );
-    assert_eq!(results[0].result_type(), ResultType::Error); // comment
+    assert_eq!(results[0].result_type(), ResultType::NonCalculable); // comment
     assert_eq!(results[3].metadata.display, "2000");
 }
 
@@ -132,7 +132,7 @@ fn sheet_with_empty_lines() {
     let mut ctx = EvalContext::new();
     let results = eval_sheet(&["x = 10", "", "y = x + 5"], &mut ctx);
     assert_eq!(results[0].metadata.display, "10");
-    assert_eq!(results[1].result_type(), ResultType::Error); // empty line
+    assert_eq!(results[1].result_type(), ResultType::Empty); // empty line
     assert_eq!(results[2].metadata.display, "15");
 }