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)/_layout.tsx

@@ -1,5 +1,5 @@
 import { Tabs } from 'expo-router';
-import { useTheme } from '@/theme';
+import { useTheme } from '@/ThemeContext';
 
 export default function TabLayout() {
   const theme = useTheme();

mobile/app/(tabs)/import.tsx

@@ -6,7 +6,7 @@ import { Pressable, ScrollView, StyleSheet, Text, View } from 'react-native';
 import { insertActivity } from '@/db/queries';
 import { PyodideWebView } from '@/extraction/PyodideWebView';
 import { extractFile } from '@/extraction/extractActivity';
-import { useTheme } from '@/theme';
+import { useTheme } from '@/ThemeContext';
 
 const FIT_EXTENSIONS  = ['.fit', '.fit.gz'];
 const OTHER_EXTENSIONS = ['.gpx', '.tcx', '.gpx.gz', '.tcx.gz'];

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();