feat(mobile): watch-folder scan button + Karoo file picker fix + docs

Import screen:
- Add "Scan for new rides" button (green) when auto_import_path is set;
  shows the configured path, lets user trigger manually in addition to
  the automatic scan on app open.
- Detect ActivityNotFoundException from DocumentPicker (Karoo and other
  stripped Android devices have no DocumentsUI app) and show a friendly
  message directing users to set a Watch directory instead.
- "No new rides found" feedback when manual scan finds nothing.

Docs (docs/mobile-app.md):
- Phase 1 marked complete with implementation notes (wheel delivery,
  timeseries workaround, source_path dedup).
- Phase 2 updated to reflect what is actually implemented (on-open scan,
  not background task) vs what remains (true background polling).
- Batch import moved from Phase 5 todo to done.
- Data model updated with source_path column.
- Known gaps section revised to remove fixed stubs.
- New Karoo sideloading section with debuggableVariants and armeabi-v7a
  troubleshooting notes.

mobile/app/(tabs)/import.tsx

@@ -3,7 +3,7 @@ import * as FileSystem from 'expo-file-system/legacy';
 import { useSQLiteContext } from 'expo-sqlite';
 import { useEffect, useRef, useState } from 'react';
 import { AppState, PermissionsAndroid, Platform, Pressable, ScrollView, StyleSheet, Text, View } from 'react-native';
-import { insertActivity, isSourcePathImported, getSetting } from '@/db/queries';
+import { insertActivity, isSourcePathImported, getSetting, useSetting } from '@/db/queries';
 import { PyodideWebView } from '@/extraction/PyodideWebView';
 import { extractFile } from '@/extraction/extractActivity';
 import { useTheme } from '@/ThemeContext';
@@ -23,6 +23,7 @@ export default function ImportScreen() {
   const theme = useTheme();
   const [state, setState] = useState<ImportState>({ status: 'idle' });
   const isImporting = useRef(false);
+  const watchPath = Platform.OS === 'android' ? (useSetting('auto_import_path') ?? '') : '';
 
   // Auto-scan watch folder on mount and when app comes to foreground.
   useEffect(() => {
@@ -38,10 +39,10 @@ export default function ImportScreen() {
 
   async function runAutoScan() {
     if (isImporting.current) return;
-    const watchPath = await getSetting(db, 'auto_import_path');
-    if (!watchPath) return;
+    const path = await getSetting(db, 'auto_import_path');
+    if (!path) return;
 
-    const newFiles = await discoverNewFiles(db, watchPath);
+    const newFiles = await discoverNewFiles(db, path);
     if (newFiles.length === 0) return;
 
     isImporting.current = true;
@@ -52,6 +53,26 @@ export default function ImportScreen() {
     }
   }
 
+  async function manualScan() {
+    if (isImporting.current) return;
+    const path = await getSetting(db, 'auto_import_path');
+    if (!path) return;
+
+    setState({ status: 'loading', msg: 'Scanning…', current: 0, total: 0 });
+    const newFiles = await discoverNewFiles(db, path);
+    if (newFiles.length === 0) {
+      setState({ status: 'done', count: 0, errors: [] });
+      return;
+    }
+
+    isImporting.current = true;
+    try {
+      await processBatch(newFiles.map(f => ({ uri: `file://${f}`, name: f.split('/').pop() ?? f, sourcePath: f })));
+    } finally {
+      isImporting.current = false;
+    }
+  }
+
   async function pickFiles() {
     if (isImporting.current) return;
     setState({ status: 'loading', msg: 'Picking files…', current: 0, total: 0 });
@@ -72,8 +93,15 @@ export default function ImportScreen() {
         isImporting.current = false;
       }
     } catch (e: unknown) {
-      const msg = e instanceof Error ? e.message : String(e);
-      setState({ status: 'error', message: msg });
+      const raw = e instanceof Error ? e.message : String(e);
+      // Karoo and other stripped-down Android devices have no DocumentsUI app
+      const noPickerAvailable = raw.includes('ActivityNotFoundException') || raw.includes('No Activity found');
+      setState({
+        status: 'error',
+        message: noPickerAvailable
+          ? 'No file picker available on this device. Set a Watch directory in Settings to import from a folder automatically.'
+          : raw,
+      });
       isImporting.current = false;
     }
   }
@@ -205,9 +233,23 @@ export default function ImportScreen() {
       <Text style={styles.body}>
         Import FIT, GPX, or TCX files — extracted on your device, nothing uploaded.
         You can also import pre-extracted BAS <Text style={[styles.code, { color: theme.accent }]}>.json</Text> files.
-        Select multiple files at once to import in batch.
       </Text>
 
+      {watchPath ? (
+        <View style={styles.watchBox}>
+          <Text style={styles.watchLabel}>Watch folder</Text>
+          <Text style={styles.watchPath} numberOfLines={2}>{watchPath}</Text>
+          <Pressable
+            style={[styles.button, styles.buttonWatch, state.status === 'loading' && styles.buttonDisabled]}
+            onPress={state.status !== 'loading' ? manualScan : undefined}
+          >
+            <Text style={styles.buttonText}>
+              {state.status === 'loading' ? 'Working…' : '↺ Scan for new rides'}
+            </Text>
+          </Pressable>
+        </View>
+      ) : null}
+
       <Pressable
         style={[styles.button, state.status === 'loading' && styles.buttonDisabled]}
         onPress={state.status !== 'loading' ? pickFiles : undefined}
@@ -232,15 +274,17 @@ export default function ImportScreen() {
       )}
 
       {state.status === 'done' && (
-        <View style={styles.success}>
+        <View style={[styles.success, state.count === 0 && state.errors.length === 0 && styles.successEmpty]}>
           <Text style={styles.successText}>
-            ✓ Imported {state.count} {state.count === 1 ? 'activity' : 'activities'}
+            {state.count === 0 && state.errors.length === 0
+              ? 'No new rides found'
+              : `✓ Imported ${state.count} ${state.count === 1 ? 'activity' : 'activities'}`}
           </Text>
           {state.errors.map((e, i) => (
             <Text key={i} style={styles.batchError}>✗ {e.name}: {e.message}</Text>
           ))}
           <Pressable onPress={() => setState({ status: 'idle' })}>
-            <Text style={styles.errorRetry}>Import more</Text>
+            <Text style={styles.errorRetry}>Dismiss</Text>
           </Pressable>
         </View>
       )}
@@ -387,10 +431,17 @@ const styles = StyleSheet.create({
   header:    { color: '#fff', fontSize: 22, fontWeight: '700', marginBottom: 12 },
   body:      { color: '#a1a1aa', fontSize: 14, lineHeight: 20, marginBottom: 24 },
   code:      { color: '#60a5fa', fontFamily: 'monospace' },
+  watchBox: {
+    backgroundColor: '#18181b', borderRadius: 10, borderWidth: 1,
+    borderColor: '#27272a', padding: 14, marginBottom: 16, gap: 8,
+  },
+  watchLabel: { color: '#71717a', fontSize: 11, fontWeight: '600', letterSpacing: 0.5 },
+  watchPath:  { color: '#a1a1aa', fontSize: 13, fontFamily: 'monospace' },
   button: {
     backgroundColor: '#2563eb', borderRadius: 10,
     paddingVertical: 14, alignItems: 'center', marginBottom: 16,
   },
+  buttonWatch:    { backgroundColor: '#16a34a', marginBottom: 0 },
   buttonDisabled: { opacity: 0.5 },
   buttonText:     { color: '#fff', fontWeight: '600', fontSize: 16 },
   statusBox: {
@@ -403,7 +454,8 @@ const styles = StyleSheet.create({
   success: {
     backgroundColor: '#14532d', borderRadius: 8, padding: 12, marginBottom: 16, gap: 6,
   },
-  successText: { color: '#86efac', fontSize: 14 },
+  successEmpty: { backgroundColor: '#1c1c1e' },
+  successText:  { color: '#86efac', fontSize: 14 },
   batchError:  { color: '#fca5a5', fontSize: 12 },
   error: {
     backgroundColor: '#450a0a', borderRadius: 8, padding: 12, marginBottom: 16, gap: 8,