refactor(frontend): modularize App.tsx into components, pages, hooks, and context (#60)

- Extract App.tsx from 764 lines to ~404 lines
- Create reusable components: MedDetailModal, MobileEditModal, ShareDialog, etc.
- Add AppContext for global state management
- Split pages: DashboardPage, MedicationsPage, SchedulePage, SettingsPage, PlannerPage
- Create custom hooks: useAuth, useMedications, useSettings, useDoses, useSchedule
- Add utility functions in separate modules
- Fix stock status logic (>30 days = green/normal)
- Fix reminder threshold calculation (use reminderDaysBefore not lowStockDays)
- Fix takenBy validation (send [] instead of null)
- Fix datetime format for blister start times (add Z suffix)
- Style 'All OK' status as green/bold

BREAKING: None - all existing functionality preserved

frontend/src/utils/index.ts

@@ -0,0 +1,8 @@
+// =============================================================================
+// Utility Functions - Barrel Export
+// =============================================================================
+
+export * from "./formatters";
+export * from "./schedule";
+export * from "./storage";
+export * from "./ics";