Add Google Tasks integration (#43)

- New GoogleTasksClient for fetching and managing Google Tasks
- Tasks appear in Timeline view with yellow indicator dot
- Tap checkbox to complete/uncomplete tasks via Google API
- Shares credentials file with Google Calendar (GOOGLE_CREDENTIALS_FILE)
- Configure task list via GOOGLE_TASKS_LIST_ID env var (default: @default)
- Supports comma-separated list IDs for multiple lists

New files:
- internal/api/google_tasks.go - Google Tasks API client

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/internal/api/interfaces.go b/internal/api/interfaces.go
index 70bba1f..aa351ab 100644
--- a/internal/api/interfaces.go
+++ b/internal/api/interfaces.go
@@ -42,10 +42,19 @@ type GoogleCalendarAPI interface {
 	GetEventsByDateRange(ctx context.Context, start, end time.Time) ([]models.CalendarEvent, error)
 }
 
+// GoogleTasksAPI defines the interface for Google Tasks operations
+type GoogleTasksAPI interface {
+	GetTasks(ctx context.Context) ([]models.GoogleTask, error)
+	GetTasksByDateRange(ctx context.Context, start, end time.Time) ([]models.GoogleTask, error)
+	CompleteTask(ctx context.Context, listID, taskID string) error
+	UncompleteTask(ctx context.Context, listID, taskID string) error
+}
+
 // Ensure concrete types implement interfaces
 var (
 	_ TodoistAPI        = (*TodoistClient)(nil)
 	_ TrelloAPI         = (*TrelloClient)(nil)
 	_ PlanToEatAPI      = (*PlanToEatClient)(nil)
 	_ GoogleCalendarAPI = (*GoogleCalendarClient)(nil)
+	_ GoogleTasksAPI    = (*GoogleTasksClient)(nil)
 )