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/Cargo.toml

@@ -6,13 +6,17 @@ edition = "2021"
 [lib]
 crate-type = ["cdylib", "staticlib", "rlib"]
 
+[features]
+default = ["network"]
+network = ["ureq"]
+
 [dependencies]
 chrono = { version = "0.4", features = ["serde"] }
 chrono-tz = "0.10"
 dashu = "0.4"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-ureq = { version = "2", features = ["json"] }
+ureq = { version = "2", features = ["json"], optional = true }
 
 [dev-dependencies]
 tempfile = "3"