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/models/event.py

@@ -1,30 +1,33 @@
-"""Pydantic v2 models for Scenar Creator."""
+"""Pydantic v2 models for Scenar Creator v3."""
 
-from datetime import date, time
+import uuid
 from typing import List, Optional
 
 from pydantic import BaseModel, Field
 
 
 class Block(BaseModel):
-    datum: date
-    zacatek: time
-    konec: time
-    program: str
-    typ: str
-    garant: Optional[str] = None
-    poznamka: Optional[str] = None
+    id: str = Field(default_factory=lambda: str(uuid.uuid4()))
+    date: str  # "YYYY-MM-DD"
+    start: str  # "HH:MM"
+    end: str  # "HH:MM"
+    title: str
+    type_id: str
+    responsible: Optional[str] = None
+    notes: Optional[str] = None
 
 
 class ProgramType(BaseModel):
-    code: str
-    description: str
-    color: str  # hex #RRGGBB
+    id: str
+    name: str
+    color: str  # "#RRGGBB"
 
 
 class EventInfo(BaseModel):
-    title: str = Field(..., max_length=200)
-    detail: str = Field(..., max_length=500)
+    title: str
+    subtitle: Optional[str] = None
+    date: Optional[str] = None
+    location: Optional[str] = None
 
 
 class ScenarioDocument(BaseModel):