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/activity/[id].tsx

@@ -6,7 +6,7 @@ import { ActivityIndicator, Alert, Modal, Pressable, ScrollView, StyleSheet, Tex
 import Svg, { Defs, LinearGradient, Path, Stop } from 'react-native-svg';
 import { useSQLiteContext } from 'expo-sqlite';
 import { deleteActivity, useActivity, useSetting } from '@/db/queries';
-import { useTheme } from '@/theme';
+import { useTheme } from '@/ThemeContext';
 
 const MAP_STYLE = 'https://basemaps.cartocdn.com/gl/dark-matter-gl-style/style.json';