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>

nginx.conf

@@ -0,0 +1,32 @@
+server {
+    listen 8080;
+    server_name localhost;
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # SPA fallback — serve index.html for all non-file routes
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    # Cache static assets aggressively (hashed filenames)
+    location /assets/ {
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+    }
+
+    # WASM files — correct MIME type + cache
+    location /wasm/ {
+        types { application/wasm wasm; }
+        default_type application/javascript;
+        expires 1y;
+        add_header Cache-Control "public, immutable";
+        add_header Cross-Origin-Embedder-Policy "require-corp";
+        add_header Cross-Origin-Opener-Policy "same-origin";
+    }
+
+    # Gzip
+    gzip on;
+    gzip_types text/plain text/css application/javascript application/json application/wasm image/svg+xml;
+    gzip_min_length 256;
+}