Add SEO improvements and CLAUDE.md

- Add visually hidden H1 for search engines and screen readers
- Add JSON-LD structured data (WebApplication schema)
- Add subtle footer tagline with keywords
- Use semantic H1 in loading shell
- Add CLAUDE.md for Claude Code guidance

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+XSLT Lab is an online XSLT editor and tester that uses Saxon for XSLT 2.0/3.0 transformations. The application consists of a React frontend and a Go backend that orchestrates Java/Saxon for XSLT processing.
+
+## Build Commands
+
+```bash
+# Full stack with Docker Compose (lightweight, no database)
+make compose-up
+make compose-down
+
+# Backend (Go)
+cd backend/src && go build -o ../server
+cd backend && ./server
+
+# Frontend (React/Vite)
+cd frontend && npm install
+VITE_BACKEND_URL=http://localhost:8000 npm run dev
+
+# Testing
+make backend-test        # Go tests: go test ./...
+make frontend-test       # npm tests: vitest
+
+# Docker images
+make backend-image
+make frontend-image
+```
+
+## Architecture
+
+### Frontend (`/frontend`)
+- React 18 + Vite SPA
+- **App.jsx** is the monolithic state container managing all workspace state, tabs, transformations
+- Monaco Editor for code editing (lazy-loaded)
+- LocalStorage persistence for workspace data (up to 3 workspaces)
+- `/lib/workspaceUtils.js` contains utility functions for parameter extraction, error parsing, XSLT manipulation
+
+### Backend (`/backend`)
+- Go/Gin HTTP server on port 8000
+- Executes Saxon via `java` command with generated args file
+- **POST /transform** - Main endpoint accepting `{xslt, version, parameters, trace}`
+- 10-second timeout per transformation
+- `/ext/` contains Java custom extension functions (uuid, date, base64, etc.)
+
+### Data Flow
+```
+Frontend (POST /transform) → Go Backend → java/Saxon → Parse trace/result → JSON response
+```
+
+### Pro Features (when VITE_GO_PRO=true)
+- Firebase authentication
+- PostgreSQL history storage
+- `/history` endpoint
+
+## Key Environment Variables
+
+**Frontend (build-time)**:
+- `VITE_BACKEND_URL` - Backend API endpoint
+- `VITE_GO_PRO` - Enable Pro features
+- `VITE_FIREBASE_CONFIG` - Firebase configuration
+
+**Backend**:
+- `DATABASE_URL` - PostgreSQL connection (Pro mode)
+- `SAXON_CLASSPATH` - Path to Saxon JARs (default in app.config)
+
+## Configuration Files
+
+- `/backend/app.config` - Backend port and Saxon classpath
+- `/docker-compose.local.yml` - Lightweight dev stack (no DB)
+- `/docker-compose.yml` - Full stack with PostgreSQL
+- `/charts/xslt-playground/` - Helm chart for Kubernetes deployment