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();

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';
 

mobile/extraction/PyodideWebView.tsx

@@ -39,6 +39,13 @@ const PY_EXTRACT = [
   'ts_path      = outdir / "activities" / (act_id + ".timeseries.json")',
   'geojson_path = outdir / "activities" / (act_id + ".geojson")',
   '',
+  '# write_activity in the installed wheel silently skips timeseries — write it directly.',
+  'if not ts_path.exists():',
+  '    from bincio.extract.timeseries import build_timeseries as _bts',
+  '    _ts = _bts(activity.points, activity.started_at, "public")',
+  '    if _ts.get("t"):',
+  '        ts_path.write_text(json.dumps(_ts))',
+  '',
   'json.dumps({',
   '  "id": act_id,',
   '  "detail": json.loads(detail_path.read_text()),',