fix: restore error handling and deduplicate goBack helper (#21)

- Replace `finally` with `catch` in [id].tsx handleSave so goBack is
  not called when updateTask/setTagsForTask fails
- Extract shared goBack helper into src/lib/navigation.ts
- Both [id].tsx and new.tsx now import goBack from the shared module

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/task/[id].tsx

@@ -25,6 +25,7 @@ import { colors } from '@/src/theme/colors';
 import { useSettingsStore } from '@/src/stores/useSettingsStore';
 import { isValidUUID } from '@/src/lib/validation';
 import { getPriorityOptions } from '@/src/lib/priority';
+import { goBack } from '@/src/lib/navigation';
 import { RECURRENCE_OPTIONS } from '@/src/lib/recurrence';
 import {
   getTaskById,
@@ -112,14 +113,6 @@ export default function TaskDetailScreen() {
     setSubtasks(result as SubtaskData[]);
   };
 
-  const goBack = () => {
-    if (router.canGoBack()) {
-      router.back();
-    } else {
-      router.replace('/');
-    }
-  };
-
   const handleSave = async () => {
     if (saving) return;
     if (!task || !title.trim()) return;
@@ -134,8 +127,9 @@ export default function TaskDetailScreen() {
         listId: selectedListId,
       });
       await setTagsForTask(task.id, selectedTagIds);
-      goBack();
-    } finally {
+      goBack(router);
+    } catch {
+      // Save failed — stay on screen so user can retry
       setSaving(false);
     }
   };
@@ -149,7 +143,7 @@ export default function TaskDetailScreen() {
         onPress: async () => {
           await deleteTask(id!);
           Haptics.impactAsync(Haptics.ImpactFeedbackStyle.Medium);
-          goBack();
+          goBack(router);
         },
       },
     ]);
@@ -193,7 +187,7 @@ export default function TaskDetailScreen() {
       <View
         className={`flex-row items-center justify-between border-b px-4 pb-3 pt-14 ${isDark ? 'border-[#3A3A3A]' : 'border-[#E5E7EB]'}`}
       >
-        <Pressable onPress={goBack} className="p-2.5" hitSlop={{ top: 8, bottom: 8, left: 8, right: 8 }}>
+        <Pressable onPress={() => goBack(router)} className="p-2.5" hitSlop={{ top: 8, bottom: 8, left: 8, right: 8 }}>
           <ArrowLeft size={24} color={isDark ? '#F5F5F5' : '#1A1A1A'} />
         </Pressable>
         <View className="flex-row items-center">

app/task/new.tsx

@@ -27,6 +27,7 @@ import { getInboxId, getAllLists } from '@/src/db/repository/lists';
 import { getAllTags, setTagsForTask } from '@/src/db/repository/tags';
 import { getPriorityOptions } from '@/src/lib/priority';
 import { RECURRENCE_OPTIONS } from '@/src/lib/recurrence';
+import { goBack } from '@/src/lib/navigation';
 import TagChip from '@/src/components/task/TagChip';
 
 const ICON_MAP: Record<string, LucideIcon> = {
@@ -64,14 +65,6 @@ export default function NewTaskScreen() {
     getAllTags().then(setAvailableTags);
   }, []);
 
-  const goBack = () => {
-    if (router.canGoBack()) {
-      router.back();
-    } else {
-      router.replace('/');
-    }
-  };
-
   const handleSave = async () => {
     if (saving) return;
     if (!title.trim()) return;
@@ -91,7 +84,7 @@ export default function NewTaskScreen() {
       for (const sub of pendingSubtasks) {
         await createTask({ title: sub, listId: selectedListId, parentId: taskId });
       }
-      goBack();
+      goBack(router);
     } catch {
       // FK constraint or other DB error — fallback to inbox
       setSaving(false);
@@ -128,7 +121,7 @@ export default function NewTaskScreen() {
           isDark ? 'border-[#3A3A3A]' : 'border-[#E5E7EB]'
         }`}
       >
-        <Pressable onPress={goBack} className="p-2.5" hitSlop={{ top: 8, bottom: 8, left: 8, right: 8 }}>
+        <Pressable onPress={() => goBack(router)} className="p-2.5" hitSlop={{ top: 8, bottom: 8, left: 8, right: 8 }}>
           <X size={24} color={isDark ? '#F5F5F5' : '#1A1A1A'} />
         </Pressable>
         <Text

src/lib/navigation.ts

@@ -0,0 +1,13 @@
+import type { Router } from 'expo-router';
+
+/**
+ * Navigate back if possible, otherwise replace with root.
+ * Shared between task screens to avoid duplication.
+ */
+export const goBack = (router: Router) => {
+  if (router.canGoBack()) {
+    router.back();
+  } else {
+    router.replace('/');
+  }
+};