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)/settings.tsx

@@ -165,18 +165,26 @@ export default function SettingsScreen() {
 
       {Platform.OS === 'android' && (
         <Section title="Auto-import (Android)">
-          <Field
-            label="Watch directory"
-            placeholder="/sdcard/FitFiles"
-            value={autoPath}
-            onChangeText={setAutoPath}
-            onBlur={() => setSetting(db, 'auto_import_path', autoPath.trim())}
-            autoCapitalize="none"
-          />
-          <Text style={styles.hint}>
-            New FIT files in this directory are imported automatically when you
-            open the app. Leave blank to disable. Requires storage permission.
-          </Text>
+          {!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"
+                value={autoPath}
+                onChangeText={setAutoPath}
+                onBlur={() => setSetting(db, 'auto_import_path', autoPath.trim())}
+                autoCapitalize="none"
+              />
+              <Text style={styles.hint}>
+                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,