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/pages/index.ts

@@ -0,0 +1,6 @@
+// Pages barrel export
+export { DashboardPage } from "./DashboardPage";
+export { MedicationsPage } from "./MedicationsPage";
+export { PlannerPage } from "./PlannerPage";
+export { SchedulePage } from "./SchedulePage";
+export { SettingsPage } from "./SettingsPage";