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/storage.ts

@@ -0,0 +1,85 @@
+// =============================================================================
+// Local Storage Utilities
+// =============================================================================
+
+import { pad2 } from "./formatters";
+
+/**
+ * Generate a user-specific storage key
+ * @param userId - The user ID
+ * @param key - The storage key name
+ */
+export function userStorageKey(userId: number | string, key: string): string {
+	return `${key}_user_${userId}`;
+}
+
+/**
+ * Get today's date as ISO string (YYYY-MM-DD)
+ */
+export function todayIso(): string {
+	const d = new Date();
+	return `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-${pad2(d.getDate())}`;
+}
+
+/**
+ * Get a date N days from today as ISO string (YYYY-MM-DD)
+ */
+export function plusDaysIso(days: number): string {
+	const d = new Date();
+	d.setDate(d.getDate() + days);
+	return `${d.getFullYear()}-${pad2(d.getMonth() + 1)}-${pad2(d.getDate())}`;
+}
+
+/**
+ * Load collapsed days state from localStorage
+ */
+export function loadCollapsedDaysFromStorage(
+	collapsedKey: string,
+	expandedKey: string
+): { collapsed: Set<string>; expanded: Set<string> } {
+	const collapsed = new Set<string>();
+	const expanded = new Set<string>();
+	try {
+		const storedCollapsed = localStorage.getItem(collapsedKey);
+		if (storedCollapsed) {
+			const arr = JSON.parse(storedCollapsed);
+			if (Array.isArray(arr)) arr.forEach((s: string) => collapsed.add(s));
+		}
+		const storedExpanded = localStorage.getItem(expandedKey);
+		if (storedExpanded) {
+			const arr = JSON.parse(storedExpanded);
+			if (Array.isArray(arr)) arr.forEach((s: string) => expanded.add(s));
+		}
+	} catch {
+		// Ignore parse errors
+	}
+	return { collapsed, expanded };
+}
+
+/**
+ * Save collapsed days state to localStorage
+ */
+export function saveCollapsedDaysToStorage(storageKey: string, state: Record<string, boolean>): void {
+	try {
+		localStorage.setItem(storageKey, JSON.stringify(state));
+	} catch {
+		// Ignore storage errors
+	}
+}
+
+/**
+ * Get theme from localStorage or default
+ */
+export function getStoredTheme(): "light" | "dark" {
+	if (typeof window !== "undefined") {
+		return (localStorage.getItem("theme") as "light" | "dark") || "dark";
+	}
+	return "dark";
+}
+
+/**
+ * Save theme to localStorage
+ */
+export function saveTheme(theme: "light" | "dark"): void {
+	localStorage.setItem("theme", theme);
+}