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/ffi_tests.rs

@@ -101,14 +101,14 @@ fn pipeline_eval_division_by_zero() {
 fn pipeline_eval_empty_input() {
     let mut ctx = EvalContext::new();
     let result = eval_line("", &mut ctx);
-    assert_eq!(result.result_type(), ResultType::Error);
+    assert_eq!(result.result_type(), ResultType::Empty);
 }
 
 #[test]
 fn pipeline_eval_comment_only() {
     let mut ctx = EvalContext::new();
     let result = eval_line("// this is a comment", &mut ctx);
-    assert_eq!(result.result_type(), ResultType::Error);
+    assert_eq!(result.result_type(), ResultType::NonCalculable);
 }
 
 // ===== Variable assignment tests (eval_sheet) =====