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, 173 insertions, 0 deletions

diff --git a/internal/api/google_tasks.go b/internal/api/google_tasks.go
new file mode 100644
index 0000000..0b4d7c2
--- /dev/null
+++ b/internal/api/google_tasks.go
@@ -0,0 +1,173 @@
+package api
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"strings"
+	"time"
+
+	"task-dashboard/internal/models"
+
+	"google.golang.org/api/option"
+	"google.golang.org/api/tasks/v1"
+)
+
+// GoogleTasksClient handles interactions with Google Tasks API
+type GoogleTasksClient struct {
+	srv        *tasks.Service
+	tasklistID string
+	displayTZ  *time.Location
+}
+
+// NewGoogleTasksClient creates a client for Google Tasks.
+// tasklistID can be "@default" for the primary list, or a specific list ID.
+// Multiple lists can be comma-separated.
+func NewGoogleTasksClient(ctx context.Context, credentialsFile, tasklistID, timezone string) (*GoogleTasksClient, error) {
+	srv, err := tasks.NewService(ctx, option.WithCredentialsFile(credentialsFile))
+	if err != nil {
+		return nil, fmt.Errorf("unable to create Tasks client: %v", err)
+	}
+
+	if tasklistID == "" {
+		tasklistID = "@default"
+	}
+
+	// Load display timezone
+	displayTZ, err := time.LoadLocation(timezone)
+	if err != nil {
+		log.Printf("Warning: invalid timezone %q, using UTC: %v", timezone, err)
+		displayTZ = time.UTC
+	}
+
+	return &GoogleTasksClient{
+		srv:        srv,
+		tasklistID: tasklistID,
+		displayTZ:  displayTZ,
+	}, nil
+}
+
+// GetTasks fetches all incomplete tasks from the configured task list(s)
+func (c *GoogleTasksClient) GetTasks(ctx context.Context) ([]models.GoogleTask, error) {
+	var allTasks []models.GoogleTask
+
+	// Support comma-separated list IDs
+	listIDs := strings.Split(c.tasklistID, ",")
+	for _, listID := range listIDs {
+		listID = strings.TrimSpace(listID)
+		if listID == "" {
+			continue
+		}
+
+		tasks, err := c.getTasksFromList(ctx, listID)
+		if err != nil {
+			log.Printf("Warning: failed to fetch tasks from list %s: %v", listID, err)
+			continue
+		}
+		allTasks = append(allTasks, tasks...)
+	}
+
+	return allTasks, nil
+}
+
+// getTasksFromList fetches tasks from a specific list
+func (c *GoogleTasksClient) getTasksFromList(ctx context.Context, listID string) ([]models.GoogleTask, error) {
+	call := c.srv.Tasks.List(listID).
+		ShowCompleted(false).
+		ShowHidden(false).
+		MaxResults(100)
+
+	taskList, err := call.Context(ctx).Do()
+	if err != nil {
+		return nil, fmt.Errorf("failed to list tasks: %v", err)
+	}
+
+	var result []models.GoogleTask
+	for _, item := range taskList.Items {
+		task := models.GoogleTask{
+			ID:        item.Id,
+			Title:     item.Title,
+			Notes:     item.Notes,
+			Status:    item.Status,
+			Completed: item.Status == "completed",
+			ListID:    listID,
+		}
+
+		// Parse due date if present (RFC3339 format, but date-only)
+		if item.Due != "" {
+			// Google Tasks due dates are in RFC3339 format but typically just the date part
+			dueDate, err := time.Parse(time.RFC3339, item.Due)
+			if err != nil {
+				// Try date-only format
+				dueDate, err = time.ParseInLocation("2006-01-02", item.Due[:10], c.displayTZ)
+			}
+			if err == nil {
+				dueInTZ := dueDate.In(c.displayTZ)
+				task.DueDate = &dueInTZ
+			}
+		}
+
+		// Parse updated time
+		if item.Updated != "" {
+			if updated, err := time.Parse(time.RFC3339, item.Updated); err == nil {
+				task.UpdatedAt = updated.In(c.displayTZ)
+			}
+		}
+
+		// Build URL to Google Tasks
+		task.URL = fmt.Sprintf("https://tasks.google.com/embed/?origin=https://mail.google.com&fullWidth=1")
+
+		result = append(result, task)
+	}
+
+	return result, nil
+}
+
+// GetTasksByDateRange fetches tasks with due dates in the specified range
+func (c *GoogleTasksClient) GetTasksByDateRange(ctx context.Context, start, end time.Time) ([]models.GoogleTask, error) {
+	allTasks, err := c.GetTasks(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// Filter by date range
+	var filtered []models.GoogleTask
+	for _, task := range allTasks {
+		if task.DueDate == nil {
+			continue
+		}
+		dueDay := time.Date(task.DueDate.Year(), task.DueDate.Month(), task.DueDate.Day(), 0, 0, 0, 0, c.displayTZ)
+		startDay := time.Date(start.Year(), start.Month(), start.Day(), 0, 0, 0, 0, c.displayTZ)
+		endDay := time.Date(end.Year(), end.Month(), end.Day(), 0, 0, 0, 0, c.displayTZ)
+
+		if !dueDay.Before(startDay) && dueDay.Before(endDay) {
+			filtered = append(filtered, task)
+		}
+	}
+
+	return filtered, nil
+}
+
+// CompleteTask marks a task as completed
+func (c *GoogleTasksClient) CompleteTask(ctx context.Context, listID, taskID string) error {
+	task := &tasks.Task{
+		Status: "completed",
+	}
+	_, err := c.srv.Tasks.Patch(listID, taskID, task).Context(ctx).Do()
+	if err != nil {
+		return fmt.Errorf("failed to complete task: %v", err)
+	}
+	return nil
+}
+
+// UncompleteTask marks a task as not completed
+func (c *GoogleTasksClient) UncompleteTask(ctx context.Context, listID, taskID string) error {
+	task := &tasks.Task{
+		Status: "needsAction",
+	}
+	_, err := c.srv.Tasks.Patch(listID, taskID, task).Context(ctx).Do()
+	if err != nil {
+		return fmt.Errorf("failed to uncomplete task: %v", err)
+	}
+	return nil
+}