Timezone correctness across the app

Five timezone bugs squashed in one release.

• 🐛 Fixed: Dashboard "Store CLOSED" mismatch — day-of-week is now resolved in the store's own timezone everywhere. Stores east or west of UTC will no longer see "closed" on the wrong day.
• 🐛 Fixed: Special dates (e.g. "Closed on Dec 25") now match the merchant's intended local calendar date regardless of the customer's or server's timezone.
• 🐛 Fixed: On first install, the app now seeds your timezone from your Shopify store settings instead of defaulting to UTC.
• 🐛 Fixed: Merchant's chosen timezone in Settings is now always preserved — earlier internal changes could overwrite an explicit choice with the Shopify default.
• 🚀 Improved: When the live "open / closed" state disagrees with what was last saved (because the open/close boundary just crossed), the storefront now triggers a background refresh so the next visitor's checkout sees fresh data.