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

package-lock.json

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@@ -11835,6 +11899,12 @@
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@@ -14731,6 +14801,12 @@
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