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>

_bmad-output/A-Product-Brief/00-product-brief.md

@@ -0,0 +1,44 @@
+# Product Brief: CalcText
+
+> The strategic foundation — why this product exists, who it serves, and what success looks like.
+
+**Created:** 2026-03-17
+**Phase:** 1 — Product Brief
+**Agent:** Saga (Analyst)
+
+---
+
+## What Belongs Here
+
+The Product Brief answers five strategic questions:
+
+1. **Why** does this product exist? (Vision & business goals)
+2. **Who** is it for? (Target users and their context)
+3. **What** does it need to do? (Core capabilities)
+4. **How** will we know it works? (Success metrics)
+5. **What** are the constraints? (Platform requirements, tech stack)
+
+Everything downstream — trigger maps, scenarios, page specs, design system — traces back to decisions made here. This is the North Star.
+
+---
+
+## For Agents
+
+**Workflow:** `_bmad/wds/workflows/1-project-brief/workflow.md`
+**Agent trigger:** `PB` (Saga)
+**Templates:** `_bmad/wds/workflows/1-project-brief/templates/`
+
+---
+
+## Documents
+
+_This section will be updated as documents are created during Phase 1._
+
+| # | Document | Status |
+|---|----------|--------|
+| 01 | Project Brief (Simplified) | Complete |
+| 02 | Brownfield Analysis | Complete |
+
+---
+
+_Created using Whiteport Design Studio (WDS) methodology_