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();