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-cli/src/main.rs

@@ -51,13 +51,10 @@ fn is_error(result: &CalcResult) -> bool {
 }
 
 fn is_empty_error(result: &CalcResult) -> bool {
-    // The engine returns CalcResult::error("empty input", ..) or
-    // CalcResult::error("no expression found", ..) for blank/comment lines.
-    if let CalcValue::Error { ref message, .. } = result.value {
-        message == "empty input" || message == "no expression found"
-    } else {
-        false
-    }
+    matches!(
+        result.metadata.result_type,
+        ResultType::Empty | ResultType::NonCalculable
+    )
 }
 
 fn error_message(result: &CalcResult) -> Option<&str> {