fix: write timeseries directly when wheel's write_activity silently skips it

Diagnosed via on-device debug: build_timeseries produces 3271 points
correctly, but the installed wheel's write_activity has a silent
exception path that skips writing the timeseries file. The workaround
calls build_timeseries directly and writes the file if missing.

Also moves useTheme import to @/ThemeContext across all tab screens.

mobile/app/(tabs)/index.tsx

@@ -5,7 +5,7 @@ import { useCallback, useState } from 'react';
 import { Alert, FlatList, Pressable, RefreshControl, StyleSheet, Text, View } from 'react-native';
 import { deleteActivities, useActivities, type ActivitySummary } from '@/db/queries';
 import { syncFeed } from '@/db/sync';
-import { useTheme } from '@/theme';
+import { useTheme } from '@/ThemeContext';
 
 export default function FeedScreen() {
   const db = useSQLiteContext();