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/static/js/api.js

@@ -1,5 +1,5 @@
 /**
- * API fetch wrapper for Scenar Creator.
+ * API fetch wrapper for Scenar Creator v3.
  */
 
 const API = {
@@ -9,10 +9,9 @@ const API = {
             opts.headers = { 'Content-Type': 'application/json' };
             opts.body = JSON.stringify(body);
         } else {
-            opts.body = body; // FormData
+            opts.body = body;
         }
-        const res = await fetch(url, opts);
-        return res;
+        return fetch(url, opts);
     },
 
     async postJson(url, body) {
@@ -33,21 +32,18 @@ const API = {
         return res.blob();
     },
 
-    async postFormData(url, formData) {
-        const res = await this.post(url, formData, false);
-        if (!res.ok) {
-            const err = await res.json().catch(() => ({ detail: res.statusText }));
-            throw new Error(err.detail || 'API error');
-        }
-        return res.json();
-    },
-
     async get(url) {
         const res = await fetch(url);
         if (!res.ok) throw new Error(res.statusText);
         return res.json();
     },
 
+    async getBlob(url) {
+        const res = await fetch(url);
+        if (!res.ok) throw new Error(res.statusText);
+        return res.blob();
+    },
+
     downloadBlob(blob, filename) {
         const url = URL.createObjectURL(blob);
         const a = document.createElement('a');