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>

app/core/__init__.py

@@ -1,28 +1,10 @@
-"""Core business logic for Scenar Creator."""
+"""Core business logic for Scenar Creator v3."""
 
-from .validator import (
-    ScenarsError,
-    ValidationError,
-    TemplateError,
-    validate_inputs,
-    validate_excel_template,
-    normalize_time,
-)
-from .timetable import create_timetable, calculate_row_height, calculate_column_width
-from .excel_reader import read_excel, get_program_types, parse_inline_schedule, parse_inline_types
+from .validator import ScenarsError, ValidationError
+from .pdf_generator import generate_pdf
 
 __all__ = [
     "ScenarsError",
     "ValidationError",
-    "TemplateError",
-    "validate_inputs",
-    "validate_excel_template",
-    "normalize_time",
-    "create_timetable",
-    "calculate_row_height",
-    "calculate_column_width",
-    "read_excel",
-    "get_program_types",
-    "parse_inline_schedule",
-    "parse_inline_types",
+    "generate_pdf",
 ]