feat: add 9 new dashboard features with export and comparison

- Add Hidden Champions tab with filterable product table
- Add Opportunity Map tab with scatter chart (supply/demand quadrants)
- Add Sales Funnel section to Overview with conversion rates
- Add Price Distribution histogram with mean/median lines
- Add Competition Score gauge (0-100) with 4 sub-metrics
- Add Excel export (3-sheet xlsx) and Print buttons to dashboard
- Add Report Comparison page with KPI diff table and brand bar chart
- Add enrichment UI to ReportList with progress tracking
- Add sidebar navigation with Karşılaştır route
- Refactor UI: Layout, Sidebar, TopBar, KpiCard, SkeletonLoader components
- Improve drill-down UX with visible pill buttons and tooltips

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

.gitignore

@@ -107,3 +107,8 @@ README_TESTING.md
 # Backend debug/analysis artifacts
 backend/*_202*.json
 backend/*_202*.png
+
+# Screenshots and test images
+*.png
+*.jpeg
+*.jpg