feat: v3.0 - canvas editor, JSON-only, no Excel, new UI

- Remove all Excel code (import, export, template, pandas, openpyxl)
- New canvas-based schedule editor with drag & drop (interact.js)
- Modern 3-panel UI: sidebar, canvas, documentation tab
- New data model: Block with id/date/start/end, ProgramType with id/name/color
- Clean API: GET /api/health, POST /api/validate, GET /api/sample, POST /api/generate-pdf
- Rewritten PDF generator using ScenarioDocument directly (no DataFrame)
- Professional PDF output: dark header, colored blocks, merged cells, legend, footer
- Sample JSON: "Zimní výjezd oddílu" with 11 blocks, 3 program types
- 30 tests passing (API, core models, PDF generation)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

requirements.txt

@@ -2,7 +2,5 @@ fastapi>=0.115
 uvicorn[standard]>=0.34
 python-multipart>=0.0.20
 reportlab>=4.0
-pandas>=2.1.3
-openpyxl>=3.1.5
 pytest>=7.4.3
 httpx>=0.27