Add Google Calendar integration

- Add GoogleCalendarClient for fetching upcoming events
- Add GoogleCalendarAPI interface and CalendarEvent model
- Add config for GOOGLE_CREDENTIALS_FILE and GOOGLE_CALENDAR_ID
- Display events in Planning tab with date/time formatting
- Update handlers and tests to support optional calendar client

Config env vars:
- GOOGLE_CREDENTIALS_FILE: Path to service account JSON
- GOOGLE_CALENDAR_ID: Calendar ID (defaults to "primary")

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

1 files changed, 62 insertions, 0 deletions

diff --git a/internal/api/google_calendar.go b/internal/api/google_calendar.go
new file mode 100644
index 0000000..836d98c
--- /dev/null
+++ b/internal/api/google_calendar.go
@@ -0,0 +1,62 @@
+package api
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"task-dashboard/internal/models"
+
+	"google.golang.org/api/calendar/v3"
+	"google.golang.org/api/option"
+)
+
+type GoogleCalendarClient struct {
+	srv        *calendar.Service
+	calendarID string
+}
+
+func NewGoogleCalendarClient(ctx context.Context, credentialsFile, calendarID string) (*GoogleCalendarClient, error) {
+	srv, err := calendar.NewService(ctx, option.WithCredentialsFile(credentialsFile))
+	if err != nil {
+		return nil, fmt.Errorf("unable to retrieve Calendar client: %v", err)
+	}
+
+	return &GoogleCalendarClient{
+		srv:        srv,
+		calendarID: calendarID,
+	}, nil
+}
+
+func (c *GoogleCalendarClient) GetUpcomingEvents(ctx context.Context, maxResults int) ([]models.CalendarEvent, error) {
+	t := time.Now().Format(time.RFC3339)
+	events, err := c.srv.Events.List(c.calendarID).ShowDeleted(false).
+		SingleEvents(true).TimeMin(t).MaxResults(int64(maxResults)).OrderBy("startTime").Do()
+	if err != nil {
+		return nil, fmt.Errorf("unable to retrieve events: %v", err)
+	}
+
+	var calendarEvents []models.CalendarEvent
+	for _, item := range events.Items {
+		var start, end time.Time
+		if item.Start.DateTime == "" {
+			// All-day event
+			start, _ = time.Parse("2006-01-02", item.Start.Date)
+			end, _ = time.Parse("2006-01-02", item.End.Date)
+		} else {
+			start, _ = time.Parse(time.RFC3339, item.Start.DateTime)
+			end, _ = time.Parse(time.RFC3339, item.End.DateTime)
+		}
+
+		calendarEvents = append(calendarEvents, models.CalendarEvent{
+			ID:          item.Id,
+			Summary:     item.Summary,
+			Description: item.Description,
+			Start:       start,
+			End:         end,
+			HTMLLink:    item.HtmlLink,
+		})
+	}
+
+	return calendarEvents, nil
+}