feat: seasonal race palette (Giro/Tour/Vuelta) + mobile picker

- theme.ts: useTheme() hook with race calendar (May–Sep windows),
  auto-detects Giro/Tour/Vuelta by date; stores override in SQLite
- All screens (feed, import, activity, tab bar) now use accent/dim
  from useTheme() instead of hardcoded #60a5fa
- Settings: Palette section with Auto/Default/Giro/Tour/Vuelta buttons
  to override the auto-detected palette for testing

mobile/app/(tabs)/index.tsx

@@ -5,9 +5,11 @@ 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';
 
 export default function FeedScreen() {
   const db = useSQLiteContext();
+  const theme = useTheme();
   const activities = useActivities();
   const [syncing, setSyncing] = useState(false);
   const [syncMsg, setSyncMsg] = useState<string | null>(null);
@@ -86,10 +88,10 @@ export default function FeedScreen() {
           <>
             <Text style={styles.header}>Feed</Text>
             <Pressable
-              style={[styles.syncButton, syncing && styles.syncButtonDisabled]}
+              style={[styles.syncButton, { backgroundColor: theme.dim }, syncing && styles.syncButtonDisabled]}
               onPress={syncing ? undefined : doSync}
             >
-              <Text style={styles.syncText}>{syncing ? 'Syncing…' : '↓ Sync'}</Text>
+              <Text style={[styles.syncText, { color: theme.accent }]}>{syncing ? 'Syncing…' : '↓ Sync'}</Text>
             </Pressable>
           </>
         )}
@@ -156,6 +158,7 @@ function ActivityCard({
   onLongPress: () => void;
 }) {
   const router = useRouter();
+  const theme = useTheme();
   const km   = activity.distance_m != null ? (activity.distance_m / 1000).toFixed(1) : null;
   const elev = activity.elevation_gain_m != null ? Math.round(activity.elevation_gain_m) : null;
   const date = new Date(activity.started_at).toLocaleDateString(undefined, {
@@ -172,14 +175,14 @@ function ActivityCard({
 
   return (
     <Pressable
-      style={[styles.card, checked && styles.cardSelected]}
+      style={[styles.card, checked && { borderColor: theme.accent }]}
       onPress={handlePress}
       onLongPress={onLongPress}
     >
       <View style={styles.cardTop}>
         <View style={styles.cardLeft}>
           {selecting && (
-            <View style={[styles.checkbox, checked && styles.checkboxChecked]}>
+            <View style={[styles.checkbox, checked && { backgroundColor: theme.accent, borderColor: theme.accent }]}>
               {checked && <Text style={styles.checkmark}>✓</Text>}
             </View>
           )}
@@ -188,7 +191,7 @@ function ActivityCard({
         <View style={styles.cardMeta}>
           <Text style={styles.cardDate}>{date}</Text>
           {activity.origin === 'remote'
-            ? <Text style={styles.remoteBadge}>cloud</Text>
+            ? <Text style={[styles.remoteBadge, { color: theme.accent, borderColor: theme.accent }]}>cloud</Text>
             : !activity.synced_at && <Text style={styles.localBadge}>local</Text>
           }
         </View>
@@ -226,11 +229,11 @@ const styles = StyleSheet.create({
   },
   header: { color: '#fff', fontSize: 22, fontWeight: '700' },
   syncButton: {
-    backgroundColor: '#1e3a5f', borderRadius: 8,
+    borderRadius: 8,
     paddingHorizontal: 14, paddingVertical: 7,
   },
   syncButtonDisabled: { opacity: 0.5 },
-  syncText: { color: '#60a5fa', fontSize: 13, fontWeight: '600' },
+  syncText: { fontSize: 13, fontWeight: '600' },
   cancelButton: {
     backgroundColor: '#27272a', borderRadius: 8,
     paddingHorizontal: 14, paddingVertical: 7,
@@ -245,15 +248,14 @@ const styles = StyleSheet.create({
     backgroundColor: '#18181b', borderRadius: 12,
     padding: 16, borderWidth: 1, borderColor: '#27272a',
   },
-  cardSelected: { borderColor: '#60a5fa' },
   cardTop:  { flexDirection: 'row', justifyContent: 'space-between', marginBottom: 6 },
   cardLeft: { flexDirection: 'row', alignItems: 'center', gap: 10 },
   sportIcon: { fontSize: 20 },
   cardMeta: { flexDirection: 'row', alignItems: 'center', gap: 8 },
   cardDate: { color: '#71717a', fontSize: 12 },
   remoteBadge: {
-    color: '#60a5fa', fontSize: 10, borderWidth: 1,
-    borderColor: '#1e3a5f', borderRadius: 4, paddingHorizontal: 4,
+    fontSize: 10, borderWidth: 1,
+    borderRadius: 4, paddingHorizontal: 4,
   },
   localBadge: {
     color: '#a1a1aa', fontSize: 10, borderWidth: 1,
@@ -268,7 +270,6 @@ const styles = StyleSheet.create({
     width: 20, height: 20, borderRadius: 4, borderWidth: 1.5,
     borderColor: '#52525b', alignItems: 'center', justifyContent: 'center',
   },
-  checkboxChecked: { backgroundColor: '#60a5fa', borderColor: '#60a5fa' },
   checkmark: { color: '#fff', fontSize: 12, fontWeight: '700' },
   empty: {
     flex: 1, alignItems: 'center', justifyContent: 'center', padding: 32,