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/components/Lightbox.tsx

@@ -0,0 +1,28 @@
+// =============================================================================
+// Lightbox Component - Full-screen image viewer
+// =============================================================================
+
+import { MouseEvent } from "react";
+
+export interface LightboxProps {
+	src: string;
+	alt: string;
+	onClose: () => void;
+}
+
+export function Lightbox({ src, alt, onClose }: LightboxProps) {
+	function handleOverlayClick(e: MouseEvent) {
+		if (e.target === e.currentTarget) {
+			onClose();
+		}
+	}
+
+	return (
+		<div className="lightbox-overlay" onClick={handleOverlayClick}>
+			<button className="lightbox-close" onClick={onClose}>
+				×
+			</button>
+			<img src={src} alt={alt} className="lightbox-image" onClick={(e) => e.stopPropagation()} />
+		</div>
+	);
+}