feat(i18n): 实现应用国际化支持，添加中英文翻译

- 为所有UI组件添加国际化支持，替换硬编码文本
- 新增useI18n钩子函数统一管理翻译
- 完善中英文翻译资源，覆盖统计、用药、通知设置等模块
- 优化Tab布局使用翻译键值替代静态文本
- 更新药品管理、个人资料编辑等页面的多语言支持

app/(tabs)/medications.tsx

@@ -1,5 +1,5 @@
-import { DateSelector } from '@/components/DateSelector';
 import CelebrationAnimation, { CelebrationAnimationRef } from '@/components/CelebrationAnimation';
+import { DateSelector } from '@/components/DateSelector';
 import { MedicationCard } from '@/components/medication/MedicationCard';
 import { ThemedText } from '@/components/ThemedText';
 import { IconSymbol } from '@/components/ui/IconSymbol';
@@ -17,6 +17,7 @@ import { Image } from 'expo-image';
 import { LinearGradient } from 'expo-linear-gradient';
 import { router } from 'expo-router';
 import React, { useCallback, useEffect, useMemo, useRef, useState } from 'react';
+import { useTranslation } from 'react-i18next';
 import {
   ScrollView,
   StyleSheet,
@@ -32,6 +33,7 @@ type MedicationFilter = 'all' | 'taken' | 'missed';
 type ThemeColors = (typeof Colors)[keyof typeof Colors];
 
 export default function MedicationsScreen() {
+  const { t } = useTranslation();
   const dispatch = useAppDispatch();
   const insets = useSafeAreaInsets();
   const theme = (useColorScheme() ?? 'light') as 'light' | 'dark';
@@ -147,8 +149,8 @@ export default function MedicationsScreen() {
 
   const displayName = userProfile.name?.trim() || DEFAULT_MEMBER_NAME;
   const headerDateLabel = selectedDate.isSame(dayjs(), 'day')
-    ? `今天，${selectedDate.format('M月D日')}`
-    : selectedDate.format('M月D日 dddd');
+    ? t('medications.dateFormats.today', { date: selectedDate.format('M月D日') })
+    : t('medications.dateFormats.other', { date: selectedDate.format('M月D日 dddd') });
 
   const emptyState = filteredMedications.length === 0;
 
@@ -178,9 +180,9 @@ export default function MedicationsScreen() {
       >
         <View style={styles.header}>
           <View>
-            <ThemedText style={styles.greeting}>你好，{displayName}</ThemedText>
+            <ThemedText style={styles.greeting}>{t('medications.greeting', { name: displayName })}</ThemedText>
             <ThemedText style={[styles.welcome, { color: colors.textMuted }]}>
-              欢迎来到用药助手！
+              {t('medications.welcome')}
             </ThemedText>
           </View>
           <View style={styles.headerActions}>
@@ -239,15 +241,10 @@ export default function MedicationsScreen() {
         </View>
 
         <View style={styles.sectionSpacing}>
-          <ThemedText style={styles.sectionHeader}>今日用药</ThemedText>
+          <ThemedText style={styles.sectionHeader}>{t('medications.todayMedications')}</ThemedText>
           <View style={[styles.segmentedControl, { backgroundColor: colors.surface }]}>
             {(['all', 'taken', 'missed'] as MedicationFilter[]).map((filter) => {
               const isActive = activeFilter === filter;
-              const labelMap: Record<MedicationFilter, string> = {
-                all: '全部',
-                taken: '已服用',
-                missed: '未服用',
-              };
               return (
                 <TouchableOpacity
                   key={filter}
@@ -263,7 +260,7 @@ export default function MedicationsScreen() {
                       { color: isActive ? colors.onPrimary : colors.textSecondary },
                     ]}
                   >
-                    {labelMap[filter]}
+                    {t(`medications.filters.${filter}`)}
                   </ThemedText>
                   <View
                     style={[
@@ -295,9 +292,9 @@ export default function MedicationsScreen() {
               style={styles.emptyIllustration}
               contentFit="cover"
             />
-            <ThemedText style={styles.emptyTitle}>今日暂无用药安排</ThemedText>
+            <ThemedText style={styles.emptyTitle}>{t('medications.emptyState.title')}</ThemedText>
             <ThemedText style={[styles.emptySubtitle, { color: colors.textMuted }]}>
-              还未添加任何用药计划，快来补充吧。
+              {t('medications.emptyState.subtitle')}
             </ThemedText>
           </View>
         ) : (