Add Slides builder, data source selector, fix cross-year labels

Features:
- Slides page: build presentations with configurable charts per slide
- Export slides as HTML zip
- Data source dropdown (Museums active, Coffees/eCommerce placeholders)
- Renamed app to 'HiHala Data' with 'Museums' as subtitle

Fixes:
- Cross-year period labels now show 'Nov 25–Jan 26' instead of misleading year
- Period display boxes updated to use smart labels

UI:
- New nav link for Slides
- Mobile nav updated
- Data source select styled to match brand

public/manifest.json

@@ -1,6 +1,6 @@
 {
-  "short_name": "React App",
-  "name": "Create React App Sample",
+  "short_name": "HiHala Data",
+  "name": "HiHala Data – Museums",
   "icons": [
     {
       "src": "favicon.ico",