fix(widget): surface unmatched task IDs instead of silently no-opping

CompleteNativeTask/UncompleteNativeTask/RescheduleNativeTask ran a plain
UPDATE ... WHERE id = ? and returned whatever error Exec gave back --
which is nil even when 0 rows match, since that's not a SQL error. A
stale or wrong id from the widget looked identical to a real completion:
HTTP 200, nothing changed in the database. Real incident: 1 of 3 widget
completeTask taps silently no-opped this way.

Now checks RowsAffected() and returns ErrNativeTaskNotFound (mirrors the
existing pattern in sqlite.go's ApproveAgentSession/DenyAgentSession).
HandleWidgetComplete and HandleWidgetReschedule surface this as 404
instead of a fake 200, so the widget can tell 'nothing changed' apart
from 'it worked.'

2 files changed, 146 insertions, 10 deletions

diff --git a/internal/store/native_tasks.go b/internal/store/native_tasks.go
index 5758407..6f8f999 100644
--- a/internal/store/native_tasks.go
+++ b/internal/store/native_tasks.go
@@ -1,12 +1,21 @@
 package store
 
 import (
+	"database/sql"
 	"encoding/json"
+	"errors"
 	"time"
 
 	"task-dashboard/internal/models"
 )
 
+// ErrNativeTaskNotFound is returned by CompleteNativeTask, UncompleteNativeTask,
+// and RescheduleNativeTask when no row matches the given id -- previously these
+// three silently reported success on a 0-row UPDATE (Exec's err is nil even when
+// no rows match), so a stale or wrong id from a caller looked identical to a real
+// completion: the HTTP response was 200, but nothing in the database changed.
+var ErrNativeTaskNotFound = errors.New("native task not found")
+
 // GetNativeTasks returns all non-completed native tasks.
 func (s *Store) GetNativeTasks() ([]models.Task, error) {
 	rows, err := s.db.Query(`
@@ -71,31 +80,62 @@ func (s *Store) UpdateNativeTask(id, content, description string) error {
 	return err
 }
 
-// CompleteNativeTask marks a task as completed.
+// CompleteNativeTask marks a task as completed. Returns ErrNativeTaskNotFound
+// if id doesn't match any row.
 func (s *Store) CompleteNativeTask(id string) error {
-	_, err := s.db.Exec(`
+	result, err := s.db.Exec(`
 		UPDATE native_tasks SET completed = 1, updated_at = CURRENT_TIMESTAMP WHERE id = ?
 	`, id)
-	return err
+	if err != nil {
+		return err
+	}
+	return checkRowsAffected(result)
 }
 
-// RescheduleNativeTask sets a new due date on a task.
+// RescheduleNativeTask sets a new due date on a task. Returns
+// ErrNativeTaskNotFound if id doesn't match any row.
 func (s *Store) RescheduleNativeTask(id string, dueDate time.Time) error {
-	_, err := s.db.Exec(`
+	result, err := s.db.Exec(`
 		UPDATE native_tasks SET due_date = ?, updated_at = CURRENT_TIMESTAMP WHERE id = ?
 	`, dueDate, id)
-	return err
+	if err != nil {
+		return err
+	}
+	return checkRowsAffected(result)
 }
 
-// UncompleteNativeTask marks a task as not completed.
+// UncompleteNativeTask marks a task as not completed. Returns
+// ErrNativeTaskNotFound if id doesn't match any row.
 func (s *Store) UncompleteNativeTask(id string) error {
-	_, err := s.db.Exec(`
+	result, err := s.db.Exec(`
 		UPDATE native_tasks SET completed = 0, updated_at = CURRENT_TIMESTAMP WHERE id = ?
 	`, id)
-	return err
+	if err != nil {
+		return err
+	}
+	return checkRowsAffected(result)
+}
+
+// checkRowsAffected returns ErrNativeTaskNotFound if the update matched no
+// rows -- mirrors the RowsAffected() check already used in sqlite.go's
+// ApproveAgentSession/DenyAgentSession for the same "silent 0-row update"
+// class of bug.
+func checkRowsAffected(result sql.Result) error {
+	affected, err := result.RowsAffected()
+	if err != nil {
+		return err
+	}
+	if affected == 0 {
+		return ErrNativeTaskNotFound
+	}
+	return nil
 }
 
-func scanNativeTasks(rows interface{ Next() bool; Scan(...interface{}) error; Err() error }) ([]models.Task, error) {
+func scanNativeTasks(rows interface {
+	Next() bool
+	Scan(...interface{}) error
+	Err() error
+}) ([]models.Task, error) {
 	var tasks []models.Task
 	for rows.Next() {
 		var t models.Task
diff --git a/internal/store/native_tasks_test.go b/internal/store/native_tasks_test.go
new file mode 100644
index 0000000..5c11d3a
--- /dev/null
+++ b/internal/store/native_tasks_test.go
@@ -0,0 +1,96 @@
+package store
+
+import (
+	"database/sql"
+	"errors"
+	"path/filepath"
+	"testing"
+	"time"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+// newNativeTasksTestStore creates a Store backed by a fresh temp sqlite DB
+// with just the native_tasks table -- enough to exercise
+// CompleteNativeTask/UncompleteNativeTask/RescheduleNativeTask without
+// running the full migration set.
+func newNativeTasksTestStore(t *testing.T) *Store {
+	t.Helper()
+	dbPath := filepath.Join(t.TempDir(), "test.db")
+	db, err := sql.Open("sqlite3", dbPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { db.Close() })
+	if _, err := db.Exec(`
+		CREATE TABLE native_tasks (
+			id TEXT PRIMARY KEY,
+			content TEXT NOT NULL,
+			description TEXT DEFAULT '',
+			project_name TEXT DEFAULT '',
+			due_date DATETIME,
+			priority INTEGER DEFAULT 1,
+			completed BOOLEAN DEFAULT 0,
+			labels TEXT DEFAULT '[]',
+			created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+			updated_at DATETIME DEFAULT CURRENT_TIMESTAMP
+		)
+	`); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := db.Exec(`INSERT INTO native_tasks (id, content) VALUES ('real-1', 'Real task')`); err != nil {
+		t.Fatal(err)
+	}
+	return &Store{db: db}
+}
+
+// TestCompleteNativeTask_UnknownID_ReturnsErrNotFound proves the 2026-07-12
+// fix: a plain UPDATE ... WHERE id = ? silently "succeeds" with a nil error
+// when 0 rows match (this is how database/sql's Exec behaves for an UPDATE
+// that matches nothing -- no error, just RowsAffected() == 0). Before this
+// fix, CompleteNativeTask returned that nil error straight through, so a
+// stale/wrong id from a caller (the Android widget, in the real incident
+// this was found from) looked identical to a real completion: HTTP 200,
+// nothing changed in the database.
+func TestCompleteNativeTask_UnknownID_ReturnsErrNotFound(t *testing.T) {
+	s := newNativeTasksTestStore(t)
+
+	err := s.CompleteNativeTask("does-not-exist")
+	if !errors.Is(err, ErrNativeTaskNotFound) {
+		t.Fatalf("expected ErrNativeTaskNotFound, got %v", err)
+	}
+}
+
+func TestCompleteNativeTask_RealID_Succeeds(t *testing.T) {
+	s := newNativeTasksTestStore(t)
+
+	if err := s.CompleteNativeTask("real-1"); err != nil {
+		t.Fatalf("CompleteNativeTask: %v", err)
+	}
+
+	var completed bool
+	if err := s.db.QueryRow(`SELECT completed FROM native_tasks WHERE id = 'real-1'`).Scan(&completed); err != nil {
+		t.Fatal(err)
+	}
+	if !completed {
+		t.Error("expected task to be marked completed")
+	}
+}
+
+func TestUncompleteNativeTask_UnknownID_ReturnsErrNotFound(t *testing.T) {
+	s := newNativeTasksTestStore(t)
+
+	err := s.UncompleteNativeTask("does-not-exist")
+	if !errors.Is(err, ErrNativeTaskNotFound) {
+		t.Fatalf("expected ErrNativeTaskNotFound, got %v", err)
+	}
+}
+
+func TestRescheduleNativeTask_UnknownID_ReturnsErrNotFound(t *testing.T) {
+	s := newNativeTasksTestStore(t)
+
+	err := s.RescheduleNativeTask("does-not-exist", time.Now())
+	if !errors.Is(err, ErrNativeTaskNotFound) {
+		t.Fatalf("expected ErrNativeTaskNotFound, got %v", err)
+	}
+}