feat: Phase 6.2 - Split ReportGenerator into DataAggregator + HtmlExporter + ExcelExporter

- Create DataAggregator service (data aggregation logic)
- Create HtmlExporter service (HTML/preview rendering)
- Create ExcelExporter service (Excel download)
- Refactor ReportGenerator to delegate to services

Tests: 101 passing (319 assertions)

app/Services/Report/HtmlExporter.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace App\Services\Report;
+
+use App\Models\Project;
+use App\Services\Report\DataAggregator;
+use Illuminate\View\View;
+
+class HtmlExporter
+{
+    public function export(Project $project, array $data): View
+    {
+        return view('reports.complete', $data);
+    }
+
+    public function exportPreview(array $data): View
+    {
+        return view('reports.partials._preview', $data);
+    }
+}