fix(mobile): gate watch directory on instance URL; skip auto-scan if unconfigured

Settings: watch directory field is hidden behind a warning if no instance
URL is saved yet, making the dependency explicit before the user sets a
path.

Import: runAutoScan silently skips (no errors) when instance URL is
missing; manualScan shows a single clear message instead of one failure
per file.

mobile/app/(tabs)/import.tsx

@@ -53,6 +53,8 @@ export default function ImportScreen() {
     if (isImporting.current) return;
     const path = await getSetting(db, 'auto_import_path');
     if (!path) return;
+    const instanceUrl = await getSetting(db, 'instance_url');
+    if (!instanceUrl) return; // silently skip — engine can't be downloaded without an instance
 
     const newFiles = await discoverNewFiles(db, path);
     if (newFiles.length === 0) return;
@@ -69,6 +71,11 @@ export default function ImportScreen() {
     if (isImporting.current) return;
     const path = await getSetting(db, 'auto_import_path');
     if (!path) return;
+    const instanceUrl = await getSetting(db, 'instance_url');
+    if (!instanceUrl) {
+      setState({ status: 'error', message: 'No Bincio instance configured. Go to Settings and enter an instance URL first — it\'s needed to download the extraction engine.' });
+      return;
+    }
 
     setState({ status: 'loading', msg: 'Scanning…', current: 0, total: 0 });
     const newFiles = await discoverNewFiles(db, path);

mobile/app/(tabs)/settings.tsx

@@ -165,6 +165,12 @@ export default function SettingsScreen() {
 
       {Platform.OS === 'android' && (
         <Section title="Auto-import (Android)">
+          {!storedUrl ? (
+            <Text style={[styles.hint, styles.hintWarn]}>
+              Configure and save a Bincio instance URL above first — it's needed to download the extraction engine.
+            </Text>
+          ) : (
+            <>
               <Field
                 label="Watch directory"
                 placeholder="/sdcard/FitFiles"
@@ -174,9 +180,11 @@ export default function SettingsScreen() {
                 autoCapitalize="none"
               />
               <Text style={styles.hint}>
-            New FIT files in this directory are imported automatically when you
+                New FIT files in this folder are imported automatically when you
                 open the app. Leave blank to disable. Requires storage permission.
               </Text>
+            </>
+          )}
         </Section>
       )}
 
@@ -326,6 +334,7 @@ const styles = StyleSheet.create({
   fieldLabel: { color: '#71717a', fontSize: 11, marginBottom: 4 },
   input:      { color: '#f4f4f5', fontSize: 15 },
   hint:       { color: '#52525b', fontSize: 12, lineHeight: 16, padding: 12 },
+  hintWarn:   { color: '#a16207' },
   row: {
     flexDirection: 'row', justifyContent: 'space-between',
     paddingHorizontal: 14, paddingVertical: 12,